Hass.Io works, Plex works, Google Assistant works.
so in theory I should be able to say something like this
Play The Big Bang Theory season 1 episode 1 from plex on my crhomecast
Or am I missing something?
As far as I know, you can’t get plex to cast via HASS. I could be completely wrong about that. Looking around it appears that some people have got it working in hacky ways. But even then they’re scripting ‘play episode x’, rather than offering a wide variety of options…
The big question mark here (and with which you don’t even need HASS) is the Google Assistant - PLEX linkage. This has been desired for over a year, and earlier in November they enabled Google Assistant built into TVs to do Plex directly via voice control, so it’s not IMPOSSIBLE that we may get this functionality into Google Home, or, like you mentioned, the Google Assistant available on phones or as roll-your-own.
Plex is simply not compatible with Google assistant yet. And since Plex never communicate nor reply on the forum, nobody knows when an integration will be released or if an integration is planned…
The only working part, is a Google assistant integrated in the Android TV Plex app, but that’s only to command the Plex app directly with your voice.
You can install Phlex on hass.io or whatever system you’re using. This allows you to do exactly this