I was thinking more along the lines of a Geo Metro with it's motor turned into a generator including removing the FWD. I figured it was easier to do that then deal with combining or manually adjusting the two throttles. You'd have to have a training session before anyone could borrow your car. The problem is still the prius inverter, 500volts straight to the motor during heavy acceleration or during highway cruising is a little harder to wire up. During that time it's easier to think the motor should be direct driving. But I'm fairly confident that running the fuel which I'm looking at
ethanol through the engine which runs between 1500rpms and 3000rpms min/max, converting that with an alternator sized for maintaining a top speed of 70mph through the inverter to the motor hooked directly is just as efficient or within 10% of the 85% efficiency of the engine hooked up to a manual transmission and FWD drivetrain.
And getnpsi, that's why they put batteries closer to the rear wheels. It's easier to put more rear bias with the heavier lead acids. Just don't oversize the battery pack or overload the rear end.
For using an AWD van, it wouldn't be anymore fuel efficient unless it eliminated idling, maintained speeds below 35mph and limiting the engine to it's optimum fuel efficient range, which is 35mph in 5th gear for the Metro but can be upwards of 50-60mph in a lower geared sports cars such as the Corvette. And then you'd have to swap the engine for a smaller one to really reap the benefits, you won't need the big v6 anymore if you use the electric motor to accelerate to 35 and you'll eliminate the need for anything shorter than 3rd and 4th gear.