Have Drupal use one database for Login, other for everything else?

I'm making a site for an MMO Game where we want the logins shared between the site and the game. We don't want to copy over the login info every time it changes, so we would like to use the Game Database to authenticate users, but have the Drupal/Site Database handle everything else.

In other words, the Game DB has a table with:
userid, username, password

and everything else would be stored in the Drupal DB that is normally stored there. The username and password would be blank in the Drupal users table.

Is it possible to have Drupal check for authentication in the GameDB without having to hack core code? Would user_login_authenticate_validate be the right hook?

We would also possibly like to fetch data from a third DB if the user doesnt currently exist, in case they haven't logged onto the new system before. This would make it so our table would be purged of any old users, but we don't have to do that. We could copy the data over (from phpbb3).

Any recommendations, opinions, etc on the process are welcome.

Thanks
Philippe