For your amusement I wondered if there were any open source watch projects that would let you hack BiP for example.
Surprisingly I found a bunch of watch projects – only one looked like it had got anywhere, and it seems dormant.
AsteroidOS – the most promising because it targeted a bunch of devices, but has gone very quiet
OSWatch – no updates after 2014
Then I found a bunch of DIY everything projects
CulBox – a kickstarter that failed to launch
Instructables – outstanding: Smooth finish, and delicate in size. My favorite by far… based on an Arduino. Notice the cool way the strap is joined to the case.