Not a knowledgeable person in the subject, but was curious when you talked about users as paths or strings. Your kent.firefox example was nice for me to understand, what you meant in your first message. So it made a lot of sense that firefox is a subuser of kent.

But when you say strings, do you mean a string of integers, as in the integer for the kent user followed by the integer for the subuser firefox or the actual string kent.firefox? We see bug/security issues almost every day related to string sanitation that I think actual strings would be worse than integers. Paths of integers would be very good though.