I’ve put some work into looking into this. As of right now, there’s no published way of getting event triggers out of the plexamp service on the raspberry pi.
A workaround for this is to use a docker application called tautulli. It has a notification system that can trigger on events such as start playback, stop playback, play, pause, etc.
It would take a bit of work and require some programming skill but if you’re interested you could set up a service on your raspberry pi that listens for webhook requests and then send out the proper RS232 package over a USB port.
I did something along these lines when the early versions of plexamp didn’t release the audio device when nothing was playing. https://forums.plex.tv/t/rpi-plexamp-release-audio-device-when-no-file-is-playing/791578/18
