As pertains to the original subject (and title) I'm sure that if someone started selling kit cars and/or building/titling kits, that were built from aluminum/titanium with a small 2 or 3 cylinder TDI and a manual or dual slip clutch transmission, modest aerodynamics and cargo space, I'm sure people would be all over it. The presumable reason that we don't see too many of those around is the red tape. Maybe cost, too, but not as much IMHO.
In Texas, any fuel other than gasoline requires NO emissions test
and depending on the inspection station, they may or may not even make sure you have all the emissions stuff you are supposed to have. So owning and operating one would be a piece of cake, but the manufacture/registration is the barrier (imo)
Edit: on another note, the way I understood it, black diesel exhaust will give you a higher EGT than the same fueling with more air. I could be confusing a strict comparison of air/fuel with timing, cause I know that advanced timing burns more fuel in the combustion chamber (as opposed to the manifold) and smokes less.