Digital calipers have an SPI like data signal that reports the current position. It's pretty easy to tap, with the caveat that the voltages are too low to trigger a +5V Teensy. Instead I enabled pull-ups on the data lines, which seem to back drive the MCU in the calipers and it seems to work without a battery installed.
On the scope Blue is the falling edge clock, yellow is 24-bits of MSB, twos-complement measurement data at 200 counts/mm. There appear to be spurious data transitions when the clock is high; it might mean something.
This could easily be cleaned up and turned into a nice Bluetooth or wired USB device.