I was thinking of scaled-up Peltier Devices that would take the heat and convert it to electricity. There is a company that makes such a device calling them Thermal Electric Generators (TEG) based in Morgantown, WV.
Thermoelectric Generator
Makes me speculate about wiring a bunch into a small woodstove and throwing it into the back of a Honda Insight or something lol. I suspect there would be power/weight ratio problems, but maybe these could be sorted out.