a friend of mine did a belly cast of me for a school project when I was about 30 weeks.
we first tried dental alginate spackled on, but it never dried properly, and was really cold. broke into chunks trying to get it off.
then we tried those plaster-gauze strips. worked much better, though you can't make a solid cast with detail from it. she decoupaged and painted the outside, it turned out beautiful. the technique we used was the same as brett's. I didn't have any trouble with my happy belly trail sticking, since we used alot of petroleum jelly (not my first choice, but these were her supplies).
happy casting! and if you have access to a digital camera, consider posting it for us to see. I love pregnancy and birth art and I can't even imagine how ripe and lovely you must look with twins!