[Bangle.js 2] modification: add buzzer, bigger battery, more switches, minus HRM

There's no schematic, no. But there is https://www.espruino.com/Bangle.js2+Technical which does contain a lot of information about it.

I think piezo may not work "as is"

Yes, you'd probably have to put a resistor across it at minimum if driving from a FET.