What would be the most power-efficient way to get data or code into something like this without having to take the back off or add a port? Maybe a light sensor, like the old Timex watches? https://en.wikipedia.org/wiki/Timex_Datalink
I guess you could do it with a peltier element and the temperature sensor, if you had a lot of time. :)
In a similar vein, the developer of the Anduril open source flashlight firmware[0][1] once investigated loading settings via light flashed from the phone using the flashlight's LEDs as photodiodes
Thanks for sharing the Timex link! I have a soft spot for hybrid analog-digital tech like that but I think a Bluetooth module (if you can figure out the antenna situation) only turned on at a certain watch face (plus a sequence of key presses) is probably the best compromise and shouldn’t be too draining if it’s fully powered down otherwise.
Other radio tech might be an even better option would need a transceiver on the PC side as well, so might not be worth it depending on how often you would be using this feature.
What about the back BEING the port? With something like the 1-wire protocol it should be possible, only activate it with say a special button combination to save on battery.
The watch has buttons, right? You could have a "data upload" screen that interprets them as 1s and 0s. I also wonder if you could add Bluetooth to the watch. That might hurt battery life, but maybe not too badly if it's toggleable?
Honestly I feel like this is the most practical way, Bluetooth modules are readily available in that form factor, just turn it on only after a certain key press on the watch (call it upload mode).
I guess you could do it with a peltier element and the temperature sensor, if you had a lot of time. :)