Miniature wireless distance sensor
Have you given any thought to LVDT (linear variable differential transformer) devices? It's been 20 years since I used them and you could get sub 30 mm devices with raw 10-bit resolution. 

We measured over a 4 mm range, it was mechanically and electrically noisy and with oversampling 1 um resolution/repeatability was achieved. Processing and analogue was difficult back then, now it would be immensely simple. May just revisit it again just to see.