I don't think I would try snow either. In googling braided rugs and snow, the results I found are for wool rugs. Apparently you lay the rug out on fresh powder (not wet snow), walk around on it, move the rug to another area, walk around on it again, and repeat until the rug isn't leaving dirty marks on the snow. Seems like it might work for a rug, which is meant to be walked on, but probably not for a quilt.
If the quilt just needs freshening up, perhaps airing it out would help. Is it stable enough to hang on a clothesline? (Never do this with a wet quilt.) It sounds like you need to get up into that attic more often and search for more treasures.
