This is really a UA forum question.
I'm presuming you mean a 12v cigarette lighter style pad? I don't see why it wouldn't be. Tough part is finding a power outlet on most planes! P.S. planes have 110v power and the premium cabins of most international configs - but I have been seeing fewer and fewer empower ports on the domestic first seats.
I imagine something like this would work fine.
http://www.amazon.com/eForCity-Power...+12v+converter
What's the posted load of your heating pad?