One possible solution would be to use a longer belt to increase wrap around the large pulley by using an idler. This will keep the belt engaged with the pulley for almost the whole way around, not giving the opportunity for a mismatch to occur.
Well, today I tried this solution, and it worked brilliantly - not a single slip!!!
I already had a longer belt (327 teeth) which I bought by mistake a while back. I also had the other skate wheel from the old suitcase. Cut a length of aluminium from a shower screen moulding, a bit of grinding, filing and drilling, and here it is:
I checked the belt tension and alignment first, adjusted the angle of one a the rollers a bit to keep the belt on track, then gave it a spin with the wheel off the ground. There was slightly more drag, my full-throttle wheel-in-air power increased from 170W to 190W, but it's well worth it. (interestingly at the end of the ride with everything warmed up this figure dropped to around 130W).
It was a little quieter than before, and has alleviated the belt rubbing on the frame in one spot as an added bonus.
Figures from my test ride down the hill and back up:
Max speed 60 km/h (37 mph)
Avg speed 42 km/h (26 mph)
Batt used 4.6 Ah
Efficiency 21.5 Wh/km (34.6 Wh/mi)
Peak power 3200W