What I'm after is restricting access on the back end.
Let's say I have 500 items in my Media Library, and only 100 of those items should be available to users with the role "student"; I want the other 400 to be off-limits to students. I don't want students to even see them in the library, much less be able to edit them, add them to posts, etc. I want them locked away from student users.
At the same time, I want students to be able to add items to the library and do whatever they want with their media items. Anything except access those 400 items to be viewable, available, editable, etc. only to users with a higher role.
So if I had my wish, there might be a feature on the Media > Media Tags screen that, would allow me to set access permissions for any tag, like this:
Items with this tag will be accessible to:
__Administrator __Author __Contributor __Editor __Subscriber __[Custom role]
I had hoped that this function is what "Roles Management" would do: set privileges for tagged media items, not just set privileges for using tags.
Thanks again for following up so quickly.