Another angle of this, some responsibility lies with the one mounting data to an "unknown" app But doing chown when necessary sound like a good idea, it would solve startup issues with RO files.
First time I used Plex btw. It's great and all that but apparently it doesn't support playing from RAR archives. Unfortunately I didn't know this before I bought the Plex app.
Hmm.. couldn't this be an opportunity for this plugin? Beat standard Plex by adding this. I'm sure you can figure out some clever solution adding RAR support here, using Kodi or some layer extracting on the fly or something. From reading several forums etc it would definitely be big news!