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.
Re:Updating STM32 Over-The-Air using ESP32
by mosselman on Tuesday May 26, 2020 @08:31AM
(#23310272)
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 ;).
try gadgetbridge, it's an OSS project that already does this for cheap fitness trackers and some watches.
I was just looking around whether it is possible to create custom apps (on phones) to sync with smart watches (generic ones from places like aliexpress). I have no experience with bluetooth, but is there anything that would prevent you from making something like that? Even if it is just for myself, I don't need to release or anything.
Do you happen to know if it is possible and where I can look?