heres my thoughts on how this can go about, to help keep functionality and control within the server owners hands.
you have:
Login Server A (centrallized login server, hypothetically based from the getmangos devs)
and Login server B (server hosted by server owner)
have a simple switch in the conf, stating either function as a dedicated login server, or communicate with login server A.
client logins into login server A, it does a check to see if that account is banned (forum ban or whatever). if passes, then client is allowed access to realm select. client selects realm and login server A then passes client onto Login server B. then Login server B checks for bans against the server, as normal, and if nothing exsists, then passes client onto realm server. this way server owner still has control over who is banned, and who isn't, keeps his functionality as well, and can choose weather to operate with the centrallized login server.
basically the switch within the conf gets checked, and if its told to communicate with login server A, then it will hold onto its check functions until signalled from login server A that a client is being passed on to it, for further confirmation. and if client is banned, then kick him to server select screen, or just to make it more annoying, kick him back to the log in screen.
idk if this seems overly complicated or not. but it seems like a viable approach. this way login server A doesn't need mysql access to login server B. login B can do all the database entries, and pass on to mangos. basically it will either keep the login server as a dedicated login server, or a relay between Login server A and realm.