I've recently had another user with the same issue and request.
I think there are just some details about you're setup that I'm not sure about, so I'm not sure exactly if Tune Span can help you without extra overhead.
When you mention "under Tune Span's control", anything in i Tunes can be moved by Tune Span.
Are you hoping to move around media files that aren't in i Tunes? But it is quite easy to add media files to i Tunes from where they are, without having them copied to the default i Tunes Media folder.
This is also not a way that I intended Tune Span to be used from the beginning, but I can see it's a natural evolution of it's functionality. It couldn't really be done in the background as gracefully as i Tunes does it in the default location, since Tune Span would have to run through the process of updating each location for i Tunes to make sure it keeps up.
It would essentially be the exact same kind of "re-span" we are talking about, just more automated, prompting the user that track info has change and the organization can be updated.
So, like if you have movies in this location, and you add a new movie, i Tunes will offer to span that new movie to the set location.