Yes, I've cogitated a lot on this one, as it is an attractive way to hybridize any vehicle and also provide range extension to a factory EV without hacking any OEM systems. I've built a trailer around an aircooled VW subframe and Kaylor-Kit but haven't run the drive system yet- am just using it as a camping trailer. (Which is another benefit of a trailer full of batteries, if you have a matching inverter.)
Weight is similar to my old aircooled VW ICE pusher trailer (now in heaven at the
Historic Electric Vehicle Museum) That pusher caused a 25% loss in EV highway range when turned off and in Neutral, but enabled unlimited cruising on gasoline (but at only 20 mpg).
400 amps/30 V through the old Kaylor Kit aircraft starter-generator would certainly give a nice 12 kW punch leaving a stoplight and also provide regen coming down from high speed, but an automated clutch would be needed to prevent windage losses during steady-state cruising. My first attempt at a clutch actuator failed during testing of the ICE pusher. Gearing would have to be selected in advance, as acceleration and regen would happen only within a particular RPM band for each (power at say, 0-1500 RPM and regen at 3000 down to 1500 RPM, with the system power neutral at 1500 RPM). Such a system would also be a natural cruise control, adding power when below synchronous RPM and regen to slow things down when the motor is overhauling...
Now get fancy with a Curtis AC drive on there, and one could add power or bring on regen at any road speed.