Sounds very interesting. I'd love to see someone try it.
I take it you're familiar with Mike D's
plug-in electric 5th wheel Honda Insight (first gen). I realize it's not the approach you're after, but the technology may be useful: He's using a compact, permanent magnet (15hp peak, I think) compact Etek motor. You could twin a pair of those.
.
.
.
Somewhat closer to your description is the not yet released Poulsen plug-in hybrid system:
Home
That approach uses "pancake" permanent magnet motors attached to each of the rear wheels @ 72v nominal.
.
.
.
Lastly, I don't know if you've seen that Coyote X has just begun a project to convert his Metro convertible into a plug-in hybrid by driving one of the front axles via (again) a permanent magnet motor at 72 volts.
See:
Geo Metro plug-in parallel hybrid EV - in progress
