I read "slosh control" as avoidance of oscillations from the initial impetus of acceleration (i.e. jerk, the first derivative of acceleration and second derivative of velocity). Acceleration will effectively need to be limited to such a rate where the off-centre weight of the passenger + seat acts as counter-torque. Applying any more torque would cause gerbilling.

That's partially true, but before the 'slosh control' the rider was thrown 90 degrees at start and stop. Afterwards, it was only maybe 30 at most.

Since even a car has some of this effect, I think his 'slosh control' is working pretty well for a prototype.

I'm sure there are ways to take it further, though.

I think it's just being more progressive about the application of acceleration. From a motorbike analogy, it's just being extremely smooth about engaging the clutch while bringing up the revs. If you pop the clutch with high revs on a bike, it too will "gerbil" - i.e. wheelie, or even loop.