Support for external IMDb/TMDb id’s has been added to the native movie agent in the upcoming 1.20.1 release which should be available for Plex Pass users later this week/early next week.
We still have the primary guid as plex:// but will we now include additional GUID id’s in the XML like so;
...
<Guid id="imdb://tt1385826"/>
<Guid id="tmdb://38050"/>
...
NOTE: In order for this to show up in the response you will need to do a Refresh all Metadata on your library after upgrading to PMS 1.20.1