I found something interesting on Wikipedia today: a hybrid turbocharger. This is a device that charges a battery with an exhaust gas turbine, and at the same time operates an air compressor using the battery to pack more air into the engine. As a benefit, the turbo lag is eliminated because the direct link between the turbine and the compressor is eliminated.
It seems to me that this device has two independent parts that can be used without each other: the exhaust gas turbine generator and the compressor. You probably need a heftier battery, too, than what is used in conventional vehicles. Thus, in my opinion this device is most suitable to hybrid vehicles.
However, hybrid vehicles have already direct electric boost, so I'm not certain that a compressor could be very beneficial. The turbine generator part, however, seems useful. There is lot of energy in exhaust gases and a turbine generator could recover some of it. In a hybrid vehicle there is good use for the recovered electricity.
So, why no production hybrid vehicles use an exhaust gas turbine generator? Is the technology too immature? Too unreliable for the goal that hybrid vehicles should have greater reliability than conventional vehicles? Or are the potential gains of such exhaust energy recovery so minor that the solution wouldn't pay for itself? Or does the Atkinson cycle used in hybrid engines mean that the exhaust gases don't have as much excess energy than in the case of the Otto cycle?