I found way to 'import' the disc .. simply insert the disc in the PC drive and then in MC right click and 'import'. That does add an entry in the library, it stores the art work in the user folder (roaming .. etc) so when you later restart the PC and run MC you can see the movie listed with the others. The only problem with this method is you have to mount each of your discs, import it, then dismount it. Would be a lot easier if you could just click 'Import' and then something like 'Add Disc'.
But with this method if you try to play it you get an error message. So not good. There are lots of tag fields available, one method would be to change the type of file to 'disc' .. that way the user can easily select file type and see their disc library titles, and when play, MC would see type disc and display a message to insert the disc.
Maybe this feature could be added to MC18 as it would really be nice to integrate both your disc storage movies with the disc library.