> What happens if the password itself - or act of unlocking - is something self-incriminating (in form, in contents, or otherwise)?

Reminds me of Ian Watkins: https://www.huffingtonpost.co.uk/2013/11/26/lostprophets-sin....

You might be able to argue that decrypting the phone's filesystem is forcing you to provide them with information which is not relevant to the case at hand but still incriminating in other ways, since a phone could reasonably be expected to hold vast amounts of unrelated days.