Ok I'm ready to send the next concept to the printer. 

Fusion 360 sheet metal didn't want to work with me. First I had to create a "flat pattern" before I could "unfold" the belt. Then, refolding either failed or created mysterium geometry like the above. I eventually just did one side and mirrored the teeth.

It seems that the most comfortable finger position is not the fingertips perpendicular to the surface, but such that the joints at the base of the fingers do the bulk of the movement work.

Thus, the hand is located lower. This, along with the shape of the thumb, means that it's much better if the thumb belt is on the bottom side.

Other things that I learned from the print was that

 The design is tolerant to finger distance and angle.

The "homing gesture" is only going to work for the top of the key since I can't feel anything at the bottom and my finger slides off.

The belt needed for the keys is 72mm (36T). Not sure how hard that's going to be to source since I've only found a 96mm (48T) belt so far. I might print it in TPU.

Renders