Didn't Nissan produce a vicsous coupling differential for their pulsars some years ago? I always wondered why they discontinued it as the improvement in the traction & handling - albeit when pushing hard - was well worth it, even if it wasn't my car....
As for AC's point about turning the direction of drive, I'm sure Dave Jackson could adapt his double universal joint for this or other purposes, as I assume he's not making double jointed supermodels full time.
Even if it wasn't possible to eliminate gears completely, the idea of reducing the weight, complexity & maintenance requirements of transmissions is highly attractive, so long as it's stability could be maintained. Friction or traction, or even a combination, are fine so long as used to best effect and the desired result is achieved.