I don't recommend finding a solution to this problem, as it will severly cripple you should an administrative password ever get out....and I know what you're thinking, no, no, they'll never figure mine out...it's secure....but it's true that the only way two people can keep a secret is if one of them is dead. I've installed Moodle for a few districts now and a day hasn't gone by yet that I haven't found out the baseball coach's password is baseball or a teacher has given his password to a T.A. and the Assistant Principal's password scribbled on his desk blotter, etc...etc...etc.... Or you go over to help a kid out on his machine and you log in just for a second to help them ....and you type your password where your username should be. Nope, something will eventually happen, and you'll want 1) Proof it wasn't indeed the admin who did it, and 2) a clue at figuring out who it was.
Cheers,
Mark