It'd be simpler to have a separate Under Duress password that behaves differently. Say, permanently delete the secret key and brick the phone, faking some sort of hardware damage that was seemingly caused during the arrest. Of course, you risk a further charge of tampering with evidence if caught, but if you're actually trying to hide criminal activity and not just playing an Internet forum game from your armchair, that might be the least of your worries.
That sounds like something they could reasonably argue was destruction (spoliation?) of evidence, and in some cases judges are allowed to tell juries to assume evidence that was destroyed is harmful to or counters the defense's arguments.
Also destruction of evidence is a crime, so you could pick up additional charges as well.
Don't play games with the law: talk to a lawyer. The law is not code, you generally aren't going to win with clever interpretation (see myriad cases where the "intent" of the people making the law is considered by the court) or "hack". If you're ever dealing with legal issues, civil or criminal, talk to a lawyer.
