One method that sort of works is to go into Tools|Options|Theater View|Appearance|Custom File Info panel.
Then Add the Comment field and place it below Description at the top. Change the Style parameter to "Text Only", and make sure the "Hide when value is empty" checkbox is checked (by default).
Now, for the Discs imports set the comment field to "*** DISC PLEASE INSERT INTO DRIVE ***"
When in Theater View now titles saved locally will not show this comment, but titles which are on Disc do. It is not the best solution because it requires that before you try to play the movie you look at the file info to the right. It would still be better if something could be added to the graphic to immediately show this is a DISC and not online.
Also, when importing all of the discs you must insert the disc, wait for the disc to register and then select Tools|Import and import a single folder. It would be a lot easier if the prompt when inserting a disc included the Import option, something like this:
Take No Action
Play Blu ray disc
Rip Blu ray disc
Import
That would make it faster to import disc titles.
Maybe there is an easier way to all this?