Ufff probably that could be possible, but it'll be needed changing some logic on security. As I can say to you is that kind of change is not trivial. And I think with actual type of grants all possible cases are covered, probably with major effors for administrator in your case, with more roles granularity, but covered. Normally making changes in a more complex logic is not a great idea.
If you've got problems setting some kind of security, explain and we'll try found some way for doing it as easilly as possible. Probably your problem could be easilly solved with other taxonomy structure.