Hmmm, I like the idea of using symlinks for this, especially if the filename of the symlink can specify which subtitle tracks go with that particular commentary. That was a major challenge we ran into while brain storming on my feature suggestion thread, with the possibility of multiple subtitle tracks per commentary for example.
It’s completely container-agnostic too, which is nice.
The only problem I see is that if you have to move your library to another storage medium, there is a lot of potential to break things with the symlinks. You could end up with multiple copies of each film on the destination, or the symlinks being copied but not directing to the also copied movie file…
For Windows Server Admins, do symlinks even exist on NTFS?
But now you’ve got me thinking again…
Maybe we’re both thinking too technical about this? Perhaps a sort of combination of both our suggestions. What if instead of a symlink, a plain text file? Name it whatever you want the commentary extra to be, like “Movie 1 (year).Director’s Commentary.conf” or “Movie 1 (year).Commentary by Actor 1 & Actor 2.conf”… or simpler like “Movie 1 (year).commentary 1.conf
Then in the text file, have a few key/value pairs… something like:
Name: Director’s Commentary
Source: Movie 1 (year).ext
Audio track: 3
Subtitle tracks:
[track name]:[track number]
[track name]:[track number]
…
Where the name is what you want Plex to display it as, source is the source file (specifying it in the file would also allow for this to work for TV Shows), which audio track is the commentary, and which subtitles are relevant. Track name would be what to display in the track chooser, like “English” or “English Audio Descriptions”, and track number would be which number subtitle track in the source it is.
I think I’ve laid it out in a yaml-like style above but anything that is easy to type out in a text editor would work.
Could probably refactor even to allow a single file per source file that then describes multiple commentaries.
Wouldn’t have to be .conf files, could be .commentary for all it matters as long as they are plaintext.
That would allow this to work in completely filesystem agnostic way too, completely removing the risk for if a user needs to migrate to new storage, and is probably easier to customise than putting it all in a filename, no?
And leave open for supporting more advanced features in the future in a backwards compatible way, for example referencing a Picture in Picture video steam to add in for commentaries that come with one would be possible, if the Plex player supported it in the future.
Anyway, food for thought 
