I build keyboards/mice for a living and also think a trackball with tap to click would be great. Intuitively I see why this would be complex to build. I would say the major paths are a physical switch or touch sensor. I see the physical switch as similar to a mouse wheel which lets you middle click it;- disassemble a mouse and see it's really quite complex to have this whole electronic component move up and down hitting lever. Now imagine trying to mount a similar system below a trackball with 3 far apart touch points and continuous movement of the ball rather than the discrete movement of the mouse wheel which can absorb some rotation (as you mention). I think a touch sensor would be a really interesting way to go, you could probably pass conduction through the right ball and sensor touching the ball, A force feedback vibration motor might also be possible.