On Login check master for user, if fail check Salve user table for user

I have a unique situation where one of our slave sites needs to allow for all the master users to authenticate, but also needs a few users unique to only that slave.

The master users experience should be the normal Bakery implementation, but the unique users should be logged into the Slave and not logged into or considered anonymous by the master. For my purpose the unique slave users are admin added only.

Conceptually, for my need Bakery should try to authenticate against the master, and if that fails then the authentication process defaults to the standard drupal login based on the local user table (and the shared cookie would not be created).

The master in our installation is using the LDAP module which contains a similar fail over option, "if NOT LDAP userm, check drupal user table".

Alternatively, conceptually I have thought about creating the "slave unique users" in the master thus allowing Bakery to function normally, in which case I would need to convince the master that those users are to be treated as if anonymous on the master.

Anyone run into this problem? Any solutions or pointers?