Would this make any sense:
 
A user asks a peer (not the one s/he normally uses) for a privledge
(such as a notebook or to do a send).  The peer establishes that the
user is not a local user *and should not be* (ie, a subscribe would
be passed to another peer).  The peer then asks the "right" peer if the
user is a subscriber there.  If a negative reply comes back or no
reply comes back, the privledge is denied.
 
This could be incredibly messy, especially as the distance between
peers increases.  Conceptually, simple, practically, Pandora's box?
 
Nick