I've noticed a couple of threads recently on how to make a quilt with a waterproof backing. I came across this blog while browsing for something else. She just used a cloth tablecloth and a vinyl tablecloth but I think you should be able to use a quilt top if you chose. She also didn't put any batting in it or quilt it. But if you look in the comment section, she was asked about washing it and her response was to wash it in the washer but air dry it.
http://thecottagehome.blogspot.com/2...-tutorial.html