Ivan, most JRA members agree that almost any sailing vessel can be converted to junk rig and be all the better for it, some being more suitable and some more problematic, but there is always a way.

A 10m catamaran is a moderately big vessel and a perhaps an ambitious proposition as a first conversion project. Therer are one or two aspects of your proposition that leave me feeling a little uncomfortable. I am not one of the experts, but as no-one else has replied I will chime in on the grounds that I have some understanding of your situation and possible mind set.

First, your suggested sail plan form, the sketched sail and mast position that you superimposed on the photograph. Without taking any measurements I can see that the proportion of sail outline ahead of the mast is a very small percentage of the total sail area. (In technical terms, the sail has "low balance").

There is nothing wrong with low balance per se but it makes no sense and probably won't work as a split junk rig. One of the most important reasons for adopting the split junk variant is that it allows high balance, normally up to 33% balance of sail area/sail outline ahead of the mast. For a low balance rig there is no point in having the split and a contiguous (unsplit) sail makes much more sense in that case.

 A couple of further  comments on the sail plan: the mast as drawn will be too short for that sail, it will need to be more like the height of the existing mast., for that sail you have drawn. (Actually, with that extreme sail, with low yard angle, I would say higher than the existing mast. Low balance and low yard angle at the same time are geometrically a bad combination for a junk rig. That's anolther topic.)

The existing rig looks like that of an auxiliary rig - that is to say, it would suggest that the catamaran is a "motor sailer" type of cruisinjg boat, perhaps 50/50 motor sailer, with a relatively low sail area. I note that your proposed sail plan shows an approximate doubling of sail area and with an extremely low aspect-ratio sail (low and wide) on a single mast. No doubt this somewhat extreme sail will push the catamaran along very nicely off the wind, but it might turn out to be not very good when sailing to windward. Maybe that doesn't matter if the boat has good engine power, which I suspect it was probably designed to have. Also it might be a little bit harder to manage than a taller narrower sail would be. If you want the rig to be low and spread out, it might be better to stick with a two mast ketch rig, but others might disagree.

Another, more serious comment about your suggested sail plan is that the sail is placed too far forward. A very rough (no measurements) diagram shows approximately the geometric centre of the original ketch rig (marked with the red vertical line).

The geometric centre of your proposed sketch sail will be very approximately on a vertical line through the mid point of its boom. You will observe that this is well-forward of the original centre, and very likely will mean the boat under sail will have "lee helm" (a tendency to steer itself away from the wind direction) which is undesirable and would probably mean that in the end you would have to add a mizzen sail at the back, to balance up the steering of the boat and give it even a little "weather helm" (a tendency to turn into the wind, which is more desirable).

Finally, mast position and support. I presume you have proposed that well-forward position for a junk mast because of interior accommodation requirements - or maybe because of structural requirements for a free standing mast. I have made a rough sketch of approximately where I think a single low-aspect-ratio split junk sail would be placed, and where the mast should probably be. I have based the sketch on my own split junk sail (an Amiina Mk 2 sail designed by Slieve) but with with double reef - a configuration which does not go very well to windward when so heavily reefed, but which gives an outline which pretty closely matches the outline of your sketched proposal. 

It has a balance of about 33%. Its not what I recommend, its just to make the point about mast placement in regard to split junk. I have no idea if placing the mast here will degrade your interior accommodation, nor whether it could be engineered in a structurally sound manner (Is there a structural bulkhead near to there?).

On a split junk sail, the sail is normally placed so that its geometric centre is approximately where the geometric centre of the original sail plan was. Interestingly. it looks as though that sail plan shape, (above) with split and 33% balance, puts the mast at about the same position as the main mast on the original ketch rig. (That might be a good omen, in regard to structure and internal accommodation (?)).

It looks a little bit odd to me, but ...    I don't think it would be very good sailing to windward, but would probably fly along down wind or on a reach, and I think it would be more manageable than your proposed low balance sail, while otherwise matching what you seem to seeking, namely a large increase in sail area, split junk, on a single, relatively short mast. Its unusual, to say the least - I would be curious to know what Arne or David T or Paul T - or Slieve - would make of it, as an option for a motor sailer. (I think I know what they will say - better go with taller narrower sail and more panels). But a short mast does have some advantages on a motor sailer which already has a lot of windage, and its less of an engineering challenge for a free- standing mast.

Personally, I would go for more sail area than the existing ketch rig currently has, but not TOO much more. and a ketch rig of low proportions does rather suit the looks of that catamaran. If the mast must go through the fore cabin, then I think either a junk ketch rig, or a lower balance single sail contiguous junk rig would suit better than SJR. You can check out Arne's suite of ready-designed contiguous sail plans here.

The expert on split junk rig (and the inventor of it)  is Slieve (you can find him on the membership list) and I strongly recommend you discuss matters with Slieve before going any further with SJR.

If you are still very keen on converting this catamaran to an extreme low-aspect-ratio single sail junk motor sailer, with mast placed through the forward cabin, you might like to look at Achieng's rig and contact her owner Nils who made this sail (https://junkrigassociation.org/page-1858517 and scroll down to Achieng April 2020)

Or Arne, who has designed and built many contiguus sails, is full of good ideas and always up for a sail design challenge. Or David T who has probably designed, built and tested more different types of junk sail than anyone.

Finally, a word about tabernacles. Normally a tabernacle for a free standing mast will be sunk into the hull (at least 10% of mast height above deck) just like a mast . (The mast itself can be sunk through the deck or sometimes wholely above deck in the tabernacle). At any rate, there is structure there, normally right down to the keel (or to the bridge deck in the case of a catamatan).  So, as I guess you will know, you need to consider the internal accommodation and how it is affected, as well as the transverse structure of the hull at that point, which will support the mast in its tabernacle at the "partners" - ie at deck level. This is routine, and not to be feared. However your mention of having a local welder make a tabernacle out of stainless steel or aluminium makes me wonder if you are contemplating something mounted entirely on the deck. This is an unusual way of doing it (it has been done) but certainly to be contemplated only with very good engineering advice. Be aware also that if your deck, as you say, is a sandwich construction, then it is not a straightforward matter to bolt substantial fittings to it.

Returning to the concept of a sunk tabernacle, I have made one with aluminium, folded into a top hat section and no welding. It is strong, cheap, simple and quick , and requires no hinge - the prototype on my little trailer boat has been a success and I have ordered a bigger one, from a local firm with a big metal folder, for my current project.

The sinking of a mast or tabernacle into the central part of a catamaran presents special problems as the minimum bury has to be 10% of mast height above deck. I am guessing that in the case of your proposed catamaran, it can be done with not too much difficulty, especially with that substantial fore cabin and very modest mast height. But don't skimp on bury or transverse strength at the partners - that's quite a  big sail area on a vessel which will be very stiff.

A wholely deck-mounted tabernacle on a flange (which might need a compression post below deck for support anyway) is an engineering challenge. Instead, some catamarans have side-by-side masts stepped into each hull, either buried or in buried tabernacles in the conventional way. The idea is proven in practice and is an interesting concept. Again I would discuss it first with someone who has actually done it. (Scan the membership list, you will find some).

Good luck. The catamaran obviously matches your dream, and almost anything is doable (if you get good advice). Presumably this catamaran ticks all the boxes for you and it does look like a nice, homely, coastal cruising and living-aboard vessel 10m not being excessively large for one person to manage and maintain, (though it is on the limit, regarding size, in my opinion). Opportunities sometimes don't come a second time (though this one has sat for a year without a buyer). If it were me, and I really wanted that boat, I would get it checked out for soundness, negotiate a very good price - and leave the ketch rig as it is for a season or two. It really looks to me like a pretty good motor sailer rig as it is. In the meantime, or during that time, do what I did: pick up an old trailer sailboat (say, 15' - 20') with a broken down rig, there are plenty around these days, at almost give-away prices, and convert it to junk, as a little project on the side.There's no harm in having two boats for a little while, the little one being a good investment of time, returning valuable experience. Keeping the original rig for a couple of seasons lets you get to know the boat and what sort of rig it REALLY requires, and gives plenty of time to consult with the experts, do the research - and then you will be able to procede with the big conversion of your dream boat with plenty of knowledge and confidence.