Is this along the lines of a "Mild Hybrid" -- although it probably wouldn't be powerful enough to run the accessories and turn the crank at the same time, like when stopped and such.
Maybe a heavy-duty clutch mechanism could be used to isolate it from the crank -- to sit/idle or EOC, and maintain A/C, Vacuum, Power Steering, etc. Hmmm. Decel charges the system and if the batts die during a trip, it's not a big deal -- de-clutch it and forget about it...
Plug it in when not in use. Good idea landspeed
RH77