Made some minor changes and printed the parts for the button prototype.
The sensor bit is some aluminum foil spray glued to the TPU pad.
The light pipe is terrible. Nice bright spot and very little actual light piping. I think I need more thickness and an optically accurate cone at the input. The printed approximation instead mimics a fresnel lens and ends up having the opposite effect. I hope I can get away with using PLA, but if not, I have some acrylic waiting in the wings.
The FSR resistance is higher than expected, and not linear. What's worse is it seems to drift with time when held down. I'm going to first try reducing the contact area to increase the force and see what happens.
The whole assembly fit together perfectly first try and it stays together.
Printing the TPU cover face down gave a really nice finish and the recessed numbers show through when lit.