You're assuming your device is still in control, and hasn't been imaged. This isn't the way a proper forensic process actually works.
More important to the issue at hand, I believe that in at least one case the reason for the passcode/phrase being "testimony" isn't so much that you're revealing what's in the locked container, but that you're demonstrating that you have access to/control over its contents. So entering your erasecode would undermine this point. A courier could not know a passphrase yet be deputized to erase the contents, but that is going to be an uphill argument.
What we really want is a proper layered steganographic filesystem, with an arbitrary number of unlockable levels. But we need an OS and apps that play nicely with that as well.
More important to the issue at hand, I believe that in at least one case the reason for the passcode/phrase being "testimony" isn't so much that you're revealing what's in the locked container, but that you're demonstrating that you have access to/control over its contents. So entering your erasecode would undermine this point. A courier could not know a passphrase yet be deputized to erase the contents, but that is going to be an uphill argument.
What we really want is a proper layered steganographic filesystem, with an arbitrary number of unlockable levels. But we need an OS and apps that play nicely with that as well.