I demand BangleJS3!

perhaps this could be made compatible with Sensor Watch daughterboards

Wow, that's such a cool idea using old Casio watch housings! But yes, something like that would be great (especially if there was room for a FPC connector).

my barometer seems to have been broken

Sorry to hear that - as above though, that post could help you. Or actually just running your watch right down (there's an option in Settings->Utils) and seeing if the full power cycle fixes it.

Still planning to write a GPS-calibrated barometric altimeter

That would be great! I have actually added seaLevelPressure to Bangle.setOptions so it should be reasonably easy to make an app that looks for a GPS event, and when it's received it then compares that with the barometer's reading and sets the value accordingly.