This would then be analogous to requiring one to produce ANY document on which you had written down the passcode for your phone. Doesn't hold water. That means you have to always memorize the passcode. It can never be recorded anywhere that you have access to outside of your secured phone.
And they have done just that in the Lavabit case ( and with Apple prior to their new system. They were demanding , and getting, peoples keys that were backed up on their servers ). If you want a good example of the government strong arming someone, watch this interview with Ladar Levison of Lavabit. I like how he initially responded to them when he was ordered to turn over his keys.
The law, as usual, is far behind the times. When these rulings on Testimony were made, there was no concept of biometric or electronic keys. If I choose to have a weak password key that I can remember in my head, I am protected. But if I decide to have a very secure password key ( long and random digits ) that I have to write down, then I am not protected. That's absurd. With thumbprint biometrics, it isn't actually the thumbprint that encrypts the data. The info from the thumbprint creates a hash key that then is used to encrypt.So if electronic keys were protected, then forcing a person to submit their thumbprint would have no effect, since all that would get them would be the hash key, which would have protected status.
Could not have said it better than you did. Thank you!