I had asked about this in another thread for an older teammate with poor circulation. Someone said it was illegal due to a self heating element (or some such). I know you can carry them onboard but might not be able to use them. My suggestion is to carry some hot water bottles. They can be filled onboard and if wrapped properly, could provide gentle heat for a few hours depending on size.

Not sure if they're allowed, but those ski handwarmers that draver mentioned would also be worth looking into. I would probably call the airline to see if they permit them in flight.