The idea of special limited categories that don't behave like other categories seems to be contrary to the overall scheme of things, and certainly would add some confusion. And the beneficiaries of such categories would seem to be extremely limited, especially considering how relativley small the entire waymarking comunity is at this point.
And like BQ, it's not clear to me why such categories couldn't somehow fit within the current structure of things. The concepts of 'global' and 'prevalent' have always been so difficult to pin down anyway - perhaps what is really needed is a better set of requirements for establishing categories in general.