I have been involver with aviation SAR for over 30 years and have been involved with about 20 crashes. Of them I can only remember three that burnt on impact, two where business jets and one was a military trainer. It seams about two thirds are not survivable crashes. Of the third that are most of the time the victims walk away. Coupled with that the general poor performance of the heaters in light aircraft I would worry more about warmth that fire protection. I like the idea of a snowmobile suit, perhaps a fleece jacket and pants with a surplus nomex flight suit over them.