Loose covers sounds like a good idea if you don't want to struggle with getting the covers back on - I put them on when damp and there was no problem, although it was a pain having to hand sew them closed. The other thing is that our rear seats have a plywood base, so the material had to be stapled on. It was really tiring trying to get the staples out to wash the covers.
Looking forward to seeing the end result - sounds lovely.