appearances matter move to switzerland

What difference would that make? The Fifth Amendment grants a general right to refuse testimony as a defendant in a criminal case. A defendant cannot be forced to surrender keys to an encrypted volume, whether hidden or not, and this has been litigated a lot already. Similar provisions exist in the constitutions of most liberal states.

The technical minutiae of hidden volumes would only matter if law enforcement and courts solely focused on the actual contents of storage devices and ignored all other evidence prosecution has already gathered to obtain a warrant to seize the defendants hardware in the first place. But a court would obviously consider all circumstantial evidence as well, and - upon hearing expert testimony about how Truecrypt can trivially create hidden volumes - would likely not find mere denial plausible anymore. The Fifth Amendment protects the defendant in this case, but it would do so too if the volume wasn't hidden.

If the Fifth Amendment didn't exist then law could just be written such that circumstantial evidence alone was already be sufficient to penalize the defendant for failing to surrender keys; the court would just be required to directly prove the existence of the volume in this case.

The whole idea of hidden volumes seems to rest on the idea that the law allows prosecution to legally force the defendant to surrender encryption keys, but then does not allow any evidence other than specifically proving that the encrypted data exists in the device contents. As a basic premise, that's rather absurd, in my opinion.