Thanks everyone for your suggestions. Currently it is cost-prohibitive to convert to a partitioned configuration but that will likely be our direction as our customer (and therefore revenue) base grows.
I have tested the $Readers field in conjunction with a secondary directory and DA and it all works nicely. Non-domain users cannot view the restricted group in the primary NAB nor person docs in the secondary NAB, and domain users can authenticate and open their dbs properly.
I had already automated the user registration process through Lotusscript from our internal support db, so it will be easy for me to manipulate the $Readers field safely by automatically including administrative groups to prevent lockout.
Thanks!