Well, the problem with a small engine is BSFC usually isn't much better than what you're getting anyway.
Anyway, you probably already know this stuff, but I might as well be thorough. The best bet for you depends on what you can do and what your routes are like.
Well, not all of it. I have experience as a mechanic and schooling in engineering but I really had not looked up any specifics on bsfc with respect to small engines and part throttle bsfc. I appreciate your input and you apparently have spent a lot more time thinking about this problem than me.
With respect to regen the only practical reasonably priced small system I found was the brushless etek with the millipack.
If you want higher voltage and regen the options are relatively expensive AC or parallel field DC motors with a little lower efficiency.
I had about talked myself into 72 volts to pull a little less amperage from the batteries and therby extend the range. The motor of choice would be a permanent magnet motor so the fields would not be powered. Nothing wrong with a series motor but the PEMG 132 seemed a bit more efficient, way lighter and sort of priced right to me. No regen though.
So let's take this idea one step further in a different direction. I noticed an older Formula 1 car that had a fan installed in the rear to suck air out from underneath to increase down force. Take this idea.... the fan provides some thrust... but maybe also it can be installed to balance the overall air pressure around the car (like air supplied from the grill) or even from the perimeter of the body where the flow changes from streaming to turbulent, but before it becomes separated, thus somehow extending the streamlined/turbulant flow longer getting more "soon to be bad" air back into the low pressure zone behind the car. When I say "bad" I mean air flow that is about to separate and cause all sorts of drag. Again, only at cruising speed and maybe only during glide. On paper you could get two beneifts... electrical energy converted into a few pounds of thrust, plus better aero.
See SAE paper 2001-01-2072. It discusses the effects of blowing air at the trailing edge of large trucks to reduce drag. Sorta what you are referring too.
OK back and just found the paper.... also found in the RC world small high thrust fans.... like up to 20 lbs of thrust at 24 volts... wondering what an additional "on demand" 10, 20, 30 ilbs of thrust would do to increasing some of the engine off glides.... fun thinking about it anyway, that is for sure.