The solution is simple – an app running in the background on the phone, listening for play/pause  rewind/fast forward commands from music playback, no matter where the commands come from – smartwatch, headphones, or anything else. The audio track playing keeps the app active, so it can work for days with the screen locked (on both iOS and Android). The "Artist Name" and "Song Name" fields on the smartwatch enable two-way communication. In my app, I display the GoPro's battery level, its video settings, and the current recording time.

I'm a cyclist, so I made it specifically for the Garmin + GoPro combination. However, this trick can be used for any other application.  The music playback interface is very robust and universal way of communication, and the benefit of having just a single mobile app is unbeatable.

The source code: https://github.com/pavloshargan/cliptracer-remote

Links to the app and video demo: https://cliptracer.com/remote/