A very old link describes how some British engineers designed a sort of turbo generator that generates electricity from the turbo being spun by hot exhaust gases. They claimed up to 6kw of power available, although that number seems a bit stretched. Seems like a neat concept, but I see no instance of this having made it to production. The link to the original article in the site is dead.
https://www.greencarcongress.com/200..._exhaust_.html
Honda is either using, or will be using, a Rankine cycle system in their hybrids that follows a similar method by using exhaust gases to evaporate water and generate electricity through steam.
If we consider that the average ICE engine is 30% efficient, or perhaps 40% efficient if it's a lean burn Honda or Atkinson cycle car, then for every 5 hp the car generates, 7-12 hp is being wasted (5.2 - 8.9 kw).
Even if such a system is only 15% efficient, that's still around a kilowatt of power being generated. And that's at 5hp; at highway speed more power could be generated.
This electrical power could then be stored and used for a variety of purposes, from a constant electrical boost to an engine to storing the electricity in batteries for home use. This would allow a more practical use of the wasted energy than trying to use the direct heat from the exhaust - which contains many toxic chemicals and therefore shouldn't be used to heat a house or other building.
One also has to consider the cost of implementing such a system - are we talking about efficiency, or economy? There is a difference between the two. Economy has to take into consideration implementation cost.