You, me, and the DOE are all interested in exhaust waste heat recovery. Unfortunately, it's only economical for long haul trucks and stationary engines at the moment.
Vehicle Technologies Program: Solid State Energy Conversion
I consider thermoelectric exhaust waste heat recovery to be far more practical than a mechanical heat engine. TEG's are much lighter, easy to package on the exhaust after the catalytic converter, and the output is already electricity.
Also, don't limit yourself to hybrids. You could just as easily (or more easily) use the system to supply 12V, so the alternator doesn't have to work as hard. Read some of the threads here on alternatorless operation to see how much gas an alternator burns.