... where Bluetooth.println() seems essential to me. It's also used for sending messages from the bangle triggering intents on android devices, like this: https://www.espruino.com/Gadgetbridge#intents
Again, I don't know exactly though.
EDIT: For another example, 'Sleep Log' app also uses interface.html.
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
Reading this: https://github.com/espruino/BangleApps#metadatajson-interface-element
... makes me think you need to do something like this:
https://github.com/espruino/BangleApps/blob/master/apps/gpsrec/interface.html#L97-L104
... where Bluetooth.println() seems essential to me. It's also used for sending messages from the bangle triggering intents on android devices, like this: https://www.espruino.com/Gadgetbridge#intents
Again, I don't know exactly though.
EDIT: For another example, 'Sleep Log' app also uses interface.html.