Slave server with invisible master server

[Spitfire14 <spitfire14@bluewin.ch>]

I have ldap clients in two separate zones. I would like that the clients 
in one of them authenticate against the slave server with no connection 
to the master. But this prevents the users to change their passwords 
because the password modification can only be performed against the 
master server with the updateref directive.
Any ideas?