I would like to see something like this too. I started a similar thread over a year ago.
MoviesAnywhere would be a good first attempt, but there are some studios like Paramount that still don’t support the MoviesAnywhere program, and virtually no TV shows are supported as far as I can tell.
I imagine this will be a complicated thing to implement. On a PC it might work pretty well, but on Roku and other platforms you’d have to decide if you want to build in a browser to stream the movie or launch an app, and what happens if the platform doesn’t have the app available. For example Google Play Movies and Vudu aren’t available on Amazon Fire TV. Then there is the encryption thing to keep people from “ripping” the stream.
It would be totally awesome if it was implemented though. Having a single database of all movies owned across platforms would be ideal.
Worst case scenario, I would settle for just the movie poster that acted as a reminder and a message saying it’s available on “whatever service”. I wonder if there is a way to manually trick Plex with a fake database entry?