I have a BF Pakit with a Sturmey Archer 4 speed & Gates belt drive. I think the belt is perfect for a folding bike. No chain grunge to foul up the other things in the travel case! Whisper quiet on the road.
To me, this issue of tight belts and hard flat fixes is overblown. I don't think it's much harder to change a flat on the Pakit than it is for my Merlin Road except when I run tubulars on the Merlin.
A Rohloff hub with belt is pretty spendy but I bet it could be set up to reasonably match the gear range on a 1x11 chain system for mountain bikes. A belt on a mountain bike would be wonderful! No low-hanging derailleur to get beat up and no problems with dirty gritty chains... maintenance would be much easier with a belt on a mtn bike.