Not for PineTime, but the bangle.js has a a 2FA TOTP app: https://banglejs.com/apps/?id=authentiwatch

For InfiniTime (the PineTime firmware), here is the issue/discussion about it: https://github.com/InfiniTimeOrg/InfiniTime/issues/310

It is pretty trivial to put this board in a Casio watch; I'm not even sure I'd call it tinkering unless you decided you want to connect the buzzer (requires soldering one bit of metal).