The HRM aspect is mostly a small binary blob that ought to be fairly straightforward to re-integrate. The far larger issue is the lack of a Bluetooth stack.

This is really unfortunate for the existing users, especially given their profound loyalty. However, more modern chips have all of this loaded in a bootloader of sorts (e.g. nRF softdevices), so the project could prevent the RF driver nonsense going forward.

They don't have permission to open-source that code: https://news.ycombinator.com/item?id=42845102

I know. That is why I'm asking which Pebble. Nothing is stopping them from writing a heart rate driver but I don't know what the plan is.