If I remember correctly, Roger Taylor put a 8in mast into Mingming II, and was a bit nervous that it would 'look' too big, but when the boat was launched and sailing, he no longer thought that.
I'm happy that my 7in mast can take anything that I can throw at it, so there's no need for the extra diameter in your case. That said, if you were to make an all-wood mast, it would be about 8in at the partners, and probably heavier than the tube that's available to you. If that tube complies with Arne's "% of the displacement" rule, it should be OK. My only other thought is that the tabernacle will have to be large to accommodate it, and take up more room inside the cabin.