First, a picture of the concept:

The flexible substrate fits over the finger using three rings created by folding the ends of the PCB over to form a ring. The arrangement of the electrodes here is such to create both the sliding as well as the touch effect.

I plan to use a Teensy for cap-sensing as they have hardware capacitive touch. However one Teensy can only do 10-11 so I will either need to use a mux or add more Teensies or find a capacitive touch device that can reasonably cover all 4 fingers for one side of the input device.

There will, however, be a challenge here. Notice that the flex PCB is already resting over a finger, so there is a chance that the reading will already reflect that the electrode is being touched! I'll probably have to figure out how to make the circuit reject the finger stimulus it's already resting on and react only to thumb taps or touches!

Before putting my ideas firmly down in a designed flex PCB for the same, I plan to use some copper foil and Kapton tape to make my own sense electrode. I've already ordered components for the same. Let's see how it goes!