Well I think we would need to, like you say, edit the script so it takes users from MyBB_users. I was going to use the MyBB forum to register users.
Could we not just change (in login.php):
PHP Code:
$query = "SELECT * FROM ".$prefix."users WHERE username = '$username'";
to
PHP Code:
$query = "SELECT * FROM "mybb_"users WHERE username = '$username'";