I'm not pretending that this is the best solution to staying warm in a hammock on a cold night, but it is A solution. As long as you don't mind carrying around a very bulky object.

I tried this at home yesterday just to see if I could get it in my hammock (HH Expedition) and with a small amount of struggling with it (no more so than other pads) I was successful.

I've got the extra wide 30" model, which I bought to insulate the bottom of cot (cot/tent actually, it's like using a hammock with the same cold backside issues, it doesn't protect you as well as a tarp, but a dog can get in it.). With this thing you won't fall off of it and get cold, and it won't slip beneath you. It's too big, and has nowhere in the hammock to slip to.

The big drawback is it's heavy and bulky, but if you're pulling a sled, it might be an option for someone.