Neoprene of that type that the Jetboil cozies are made of would work OK depending on what type of heating you were doing. Open flame (such as a wood fire) would probably be bad although you might be able to slide the cozy off during heating and then slide it back on afterwards.

The problem I see with using a cozy on a canteen cup is the cup's handles. You'd have to cut out a pretty big swath of the cozy in order to fit it onto the cup. I wonder how stable it would be although perhaps you could have a velcro attachment that fastened above the handles once the cozy was put into place. Seems like kind of a kludge.

HJ