Re the hub, I've been using mine on my commuter year round for 5 + years..
just got a second bike with one, a Bike Friday..
no belt drive as the frame folds behind the BB for 20" wheels.
It has a spring tensioner .

It is an option on their 349 wheel bikes as rear section is a unit,
fold is including the BB, hinge ahead of the BB.
IGH dropout swings rearward to tension drive, chain or belt.

Rohloff has drawings to manufacture sliding dropouts ,
their sliding track spec was angled at 20 degrees.
I see that as moving in a direction to not move the brake shoes away from the rim

and the weight of the bike, when one has loosened the bolts,
tensions the drive chain, because the slide has slope,
so perhaps also valid for the belt scheme.