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I think putting your friction drive behind the tyre won't work, it will only take a small amount of suspension movement before contact pressure is lost, or it will take a lot of excess pressure to prevent this. Think about a steep hill start, suspension compresses, drive looses friction, you go no where. You also have to factor in the extra rolling resistance and possible bearing/ suspension wear from fore/aft loads it wasn't originally designed for. You could design all sorts of mechanical methods to compensate for wheel travel, but then it gets pretty complex. Plus in the EU everything you do has to be type approved, no?
The sprung separate wheel on the road is a far simpler idea to pull off, it's been done at least a couple of times.
Far easier to buy something like a Fiat Panda 4x4, remove the drive shaft and add a motor to the back.
I'd consider a small powered aero trailer, you'd gain hybrid capacity, improve aero and perhaps even have a bit more cargo space. Maybe even a swivel wheel single wheel type unit.
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