... one that connects to Plex..... So far I have only attached one device to PLEX with mixed results (HD Homerun for live TV).... works great for recording and viewing on IOS, not on TVOS or any other PLEX client.. When you mention "connects to Plex" ... "plugin can control" ... are you talking about physical networked devices added to the Plex Server or Plex plugins within the Plex server?
The Plex Media Manager (Indigo) Plugin can remotely control many different Plex clients -- for instance, if I am watching a Plex-based movie/audio/tv show on my Android tablet or Roku, I can actually stop/pause/play the client from the Plex plugin in Indigo.
So what I am suggesting is that you get another small piece of hardware -- be it a Raspberry Pi or Roku or whatever -- which has a Plex client and can play audio. Now, obviously I don't know your actual audio equipment and distribution, but I'm assuming you have an Audio In of some kind to which the client would be connected for distribution. The only thing you would have to be careful of in this setup (and I little research should answer) is that some clients -- like the Roku 2's Plex Channel allow control but is limited in what you can "push" to it. IIRC many clients will allow pushing, say, a playlist to it as long as the Plex client / channel is up. That is why one would need a dedicated device to consume the audio.
I'm not positive I explained that well or cleared it up. If needed I can take some more time tonight to detail it better.
Adam