Erik, can't you just add your own custom field with your own types and replace "media sub type" with your own in theatrer view ?
The other problem with doing this is that you give up data portability (such as when moving files to an iPod), and the special bookmarking features in MC. If you want to preserve both of these things, and still have flexibility, then you need to duplicate the tag into your own field and remember to manually manage BOTH fields. That's no good.
I really don't feel like it has ever been adequately explained why this field can't be "manually unlocked". I can understand why you don't want novice users mucking about with it, but why can't we have a "I know what I'm doing, so unlock this thing" button, which gives a pop-up with proper "this is unsupported and might kill your pet parrot" warning dialog? That, or set it so it can be "additive-only"... Meaning: You can add (and then later delete) your own "Media Sub Types" but not delete or alter any of the stock ones.
All of the explanations for why this isn't possible have always amounted to "trust us, we have a reason, and we know what we're doing". Well, I'm sorry, JRiver folks... I do generally trust you, and you do generally know what you're doing, but there have been people consistently asking for this for
years and years. And that means that
something is wrong in the basic assumptions that led to the design decision.