How about using Moonpi's idea....make the out-door quilt out of nylon. At least the top and backing could be nylon, which would stand up to the weather. Use something strong with a lot of polyester in it for the batting.
Has anyone seen the bolts of fabric Wal-Mart is carrying that are labeled something like 90% Polyester and 10% Nylon? This would work for an outdoor quilt.
June