This is an idea I keep dreaming about as well. Realistically, I have enough on my plate that I probably won't ever do it, but it's fun exercising the synapses...
Have you seen Mike D's
e-5th wheel on his Insight? (I'm assuming so).
He started out with a child's bike frame/wheel and an old wheelchair motor, if I recall correctly. Though he's since upgraded to an Etek PM motor and a beefier scooter type wheel/tire.
I missed out on a used 24/36v 400A golf cart controller on eBay today that would have been ideal for testing something like this. It went for 30 bucks. (DRAT!)