[nickff]

There are no hardware obstacles to doing that, and BLE (BT4 and 5) support custom protocols. That said, I think you will find that working with the proprietary operating systems and architectures on watches and phones is much more difficult than you anticipate.

[mosselman]

I figured it is most likely too difficult. For the amount of work it would take I could just buy an Apple watch and be done with the privacy concerns I have with cheaper trackers, but once the seed of an idea is planted it starts having a life of its own.

I think this is an idea I shouldn't water though ;).