I know it is a bit late, but you should also take a look at the external_auth.php script. It is core implementation a plugin can use to enable authortie outside of zenphoto to be used for user id and autentication. Unfortunately, the sole example of its use is not publicly available, but it is the basis of a plugin which uses a bbs system's authentication.
In this case there are no Zenphoto users created in the database. All rights come from the external source which is preferable, since otherwise it violates the addage that if you have two copies of something then one of them will be wrong.