They private key is the encrypted information they want, you have the key, give it up or go to jail. Even if the law won't allow this atm, it's a small technical change and it's all the same.
Wouldn't they have to prove that you still have access to the keys? And wouldn't it be fairly easy to devise new techniques to keep anyone from finding out exactly how many coins you have? Plus you could do some really crazy things with multi-sig. Like lock your coins from being spent for a time, or even set it up so that even if they get your password, they can't do anything with your coins without catching or finding someone else.