It's certainly interesting, and might be portable, but there are some differences we'd certainly need to deal with before it will be usable.
The PineTime doesn't have the GPS so that's out.
The bangle has 3 hardware buttons plus 2 soft buttons. The hardware buttons are integral to the UI operation, so we'd need to reimplement to use soft buttons. They're also used in the reset process - not sure what we'd do about that.
Sensors/screen - not sure if they're compatible, so might need a different set of drivers.
Bangle.js technical details
Screen controller: same controller but different mode (parallel for Bangle.js, SPI for PineTime)
Touch controller: different - plus the 'touch left/right' have hardware pins so aren't really 'soft' buttons
Accelerometer: different
HRM: different
Magnetometer: only on Bangle.js
Flash: possibly similar
So a port may be possible but isn't going to be trivial, and unless they're on board with the reduced feature set device the app compatibility may be a pain. It seems worth investigating though.