RE: RC square-rigger, 1:96 scale USS Constitution
Cool project a lot to fit in that small package but I wanna see it done!
For sailcloth, there is (or at least used to be) lightweight (3/4oz) dacron that's great for models. I don't live within a hundred miles of the nearest sail maker, so that kind of stuff is hard to come by here. Another decent choice is Tyvek. It's light and it's made of bonded spun polyester fiber, so it's very tough and easy to sew.
I hadn't though to look at how people attach square sails, but I'd think that something similar to full size practice COULD do the trick an eyelet/cringle every so often along the head of the sail and bind it to the yard with the same kind of stuff you use for sheets. I'd try it with a single piece of line running the whole length of the head rather than the individual piece for each cringle on a full sized sail. But then, there may be better ways than what I'm thinking, too.
The ship rigged models I've seen weren't trimming both square sails and fore-and-aft sails... but that doesn't mean it couldn't or shouldn't be done. I'd think it could be quite a challenge in the amount of space you'll have. I'd love to see how it all turns out. Not sure if the standard servos will pull it all I'm guessing maybe 500 or 600 squinches on the square courses, but it should be a somewhat balanced (I think) so it might just work. They're mighty low priced and later you can go to at least 3 or 4 times as much strength in the same footprint if you try them and they doin't work out.
Meanwhile, not really wanting to start off on a tangent, but you wouldn't happen to have a set of lines for a 3-island tramp would you?