This is basically the idea behind TrueCrypt hidden volumes and similar: there should be no way for the police to prove that there exists additional volumes which you have not decrypted for them.

But afaik, no case in the United States so far has involved an order to just "decrypt all your files". In all the cases I have heard about, they had something specific that they wanted the key for, and they had separate evidence that the defendant knew the key. In that case no technical solution can help you.