I came across this versatile poncho that is first most a poncho, a hammock, a ground sheet, a weather shield and a rainwater catcher. See the following link for some more info:
If it can be used as a hammock I can not see why it can't be used as a underquilt protector as well. I certainly like versatility.....
O yes, if you want to order, do not bother right now. I ordered today, but it was cancelled as they are currently out of stock. Sounds as if they will have stock sometime in the new year.