Howard, as David pointed out, if you are agile you can go over the bottom as the dinghy rolls, and bring it up again without getting your feet wet - with a bit of practice. And if doing a half-roll, put weight out on the horizontal board and scramble back in without having to get in the water. But its more of a stunt really, and only when single handed. You and/or any others present, will most likely end up in the water.
As for remaining in the confines of the boat during a capsize - initially I thought that was where the "self-righting dinghy" idea was heading. Next thing we will be fitting seat belts.
[If you tried climbing up onto the bottom of a knocked down Scamp, from within the boat, I think you might just pull it into the fully upside down position. That's just a guess. In any case, you will be wet already so probably easier to go round the outside. To do the climbing over the top trick, you have got to be in control of the capsize and a little bit ahead of events].
I don't know about open transom, I suppose it would be easier to board if you had something to grab, but I have a vision of it disappearing into the distance, tend to lose things overboard enough as it is - and think maybe there are reasons why most boats have transoms. Good thought - it is good to think about these things - but I would pass on that idea, I think. I am planning to put climbing steps on my scow rudders, starting with the end plate, as it is the under-water step, usually not there, which makes it most difficult to get started (this remark applies to Arne's angle-out ladder too) - the rudder steps seem a good idea to me but I have never tried it yet.