Actually you can do this now. Like, you used to only be able to ask Siri to play podcasts in Apple podcasts. But now you can say “Play This American Life in Pocket Casts”. For music you can say “Play Cowboy Junkies” and it will default to the last music app you used, or you can say “Play Cowboy Junkies in Pandora”. I often have to say “Play Rush in Apple Music” because it keeps trying to use Pandora.
I think you may have to implement the Siri API in your app to support this, and I don’t know if Plex has done that.
