I'm thinking a Staton friction kit on the rear wheel....?
I've been contemplating on this thing a good bit, and I have a design mostly worked out. As I see it, the 20" rear (steering) wheel would be powered by, say, a Tanaka 32cc engine via cogged belt, a la GEBE's system. The engine is mounted co-planar with the rear wheel, above it, on a mounting yoke that pivots with the wheel.
The front two wheels would be powered via pedal chain back to a multi-speed internally geared hub, then forward again to a pair of independent "half-axle" jackshafts, with fixed geometry final chain loops to the actual stub axles.
There is a method to the apparent madness, actually - I want the seat as low as I can reasonably get it, and with this design the seat is at 11 1/8" off the ground. A variable length crank boom allows for different height riders. This way, with 26" front wheels, the CoG is well below axle height, and trailing the main axle height by about 16".
With a 30" wheelbase width and a wheelbase length of just 34", this thing will turn on a dime and give you nine cents change - hopefully without flipping over.
I'll have a sketch or three to post soon.