As long as we're in the half-bakery here, half-baking silly ideas...
Begin with a 4wd or AWD vehicle, let's say a Forester for purpose of discussion.
Manual transmission Foresters do not have a "50/50" center differential so the front wheels are always directly driven, even if the rear driveshaft is removed entirely.
Remove the rear driveshaft, and fit a
Tesla-style viscous adhesion air motor to the yoke in its place. Plumb the air motor into the engine exhaust with a transmission-linked bypass, so that there is negligible pressure when in neutral or reverse.
Now, you can also plumb an electric blower into the exhaust as well, between that bypass and the air motor. Clearly it would have to be a fairly large capacity one, but I've read that viscous adhesion devices can, in theory, see better than 90% efficiency so it wouldn't be too drastic. The electric blower would then supplement forward motion on battery power as well as the "recaptured" exhaust velocity - with relative simplicity and fault tolerance.
Pity no-one that I'm aware of mass produces viscous adhesion air motors - though similar devices are used as slurry pumps in mining.