I have never personally tried this but I found these online.
Grabber Body Warmer
They say they put off heat for 12 hours and they are much bigger than the hand warmers and at $2.50, fairly reasonable. I know its not a lot of heat but i was thinking if you activated one and placed it below your plant, the heat would rise into the plant and as long as it wasnt really cold, it might be enough to ward off frost.
Like i said i've never tried it but it would be a good experiment.