I think I saw somewhere there was someone trying to work out a belt shift across pulleys. I think it ended with something like a outer and inner shaft that would switch two sets of pulleys. I'm not sure why a derailleur for a chain can't be also made for belt.
My 26 inch wheel drive I have a 3hp not with 22:1 ratio and I think it is max at about 10 mph. The prior 14:1 ratio was just to high a ratio to get up steep hills without a long run up but I think it was doing on the flat about 25mph.