The way I defeat that is simple- I use a packcloth bag cover (homemade) in summer to keep the outside of the bag clean and mostly dry in my hammock (pine sap sucks). In winter, I stuff my pad down in there with bag. If it all one peice, you might roll over completely, but squirm off.

This winter, I'm thinking I'm going to try doing my hammock, about 12" off the ground, with an A-framed tarp that has snow piled on it. It's worth an experiment.