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(I could be corrected on any of the following by many) These are thoughts, impressions and recollections...
I think the problem with exhaust heat is how do you tranfer it to do work? Increasing back pressure would make the ice even more inefficient. The amount of heat you could "capture" from the exhaust would be negligible, I'd assume. I think after things like motor inefficiencies and the extra weight, you wouldn't even be able to run an alternator off most cars exhaust using some kind of steam engine. Possibly a tiny turbine? I think the efficiencies of most turbines is not scalable, the smaller you get the less efficient they become.
Maybe a ceramic motor would help create more useable exhaust heat. I'm pretty sure an ice's maximum efficiency is linked to a temperature differential, that is, a very very hot engine would be a lot more efficient than a "cool" engine with the same outside temperature. So sapping heat from the engine probably would not get you a net gain.
I believe the same thing can be said for most hybrids. Using an ice to charge a battery (or hydraulic accumulator) for propulsion would be less efficient than just using a direct motor to propulsion transmission. Except for gains made in engine off (coasting or stopped) time. Which would be possible with most any car now.
Sorry for the tangents...
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