Could this be a potential solution?
It gives the ability to send each other messages if you opt in. Granted these are messages around a given show but while someone is watching a movie or TV show the admin could send a message to say that there’s planned maintenance in a few hours and the server will be offline for about 30 minutes.
Current implementation of messaging in Plex
If it’s not exactly that, I don’t imagine it would take a major effort to create something that isn’t based on a particular movie or show allowing admins to just broadcast a message to all connected users.
