Interesting idea. A couple of years ago I found some material that you could cut and attach to a battery pack to heat an area (insoles, hand warmers, etc) but I just did a Google search and couldn't find it.
If you heat your feet for really cold winter riding, make sure your feet can breath out the perspiration. Last winter I stuck an air activated foot warmer in each of my shoes. After about 7 miles my feet started getting REALLY cold. I stopped and took the foot warmer out and sure enough, the foot warmer froze into a solid block of ice! Plus, my socks were wet for the rest of my ride to work!
I'll probably try something similar again this year but I'll need to change my strategy a little. Still, awesome idea!
Jerry H