The chemical hand warmers that REI sells are worth a try. They might sound kind of wimpy, but they are inexpensive, lighweight, and really work. I took some to Alaska this past winter in order to warm my hands while dealing with a medium format camera on a ski trip. I ended up using them at night in my sleeping bag. When it got really cold (< -40F) and I couldn't sleep, I would place one at the foot of my bag and one on my chest. They made all the difference between shivering and a night of semi-sleep. My guess is that they would more than make up for their weight on a forced bivy.
-Steven