I can feel the RSI already. I was excited that this might have been a weird hybrid between a mouse and a trackpad where the tracking surface goes on your fingertip, and everything becomes a trackpad.
I'd want it out of the way for typing and other tasks.
The thumbnail is probably a good place to mount because it's not touching skin for long periods of time, or blocking the pad of fingers.
I'd test a pointing stick "eraser" or a trackball of some sort (because capacitive touchpad style won't work on everything) on the fingernail tip.