@andreas and Michael
But you *would* want to sit in a solar-powered 70mph mobility scooter...? Hmmm.
I reckon if most of the weight's down nice and low, it wouldn't be a problem. Remember you've got all that nice gyroscopic stability from the wheels too. And the energy to slide a balance weight is likely to be fairly small compared to the energy used by the wheels. Hell, the batteries could even be the balance weight, so no need for extra weight in the system apart from an accelerometer and a small motor. Also remember that what you lose in balancing the vehicle, you gain in significant reductions to rolling resistance and frontal area.
Interestingly, the rules don't say anything about the number of wheels required, so a two-wheeler does appear to be a legitimate design. (A unicycle is only prohibited because the regs say that braking has to be appplied to two wheels...)