I have some definite concerns with this. If producing the key via government coercion is legal, where do we stop? Right now, we have (admittedly) very crude technology that lets us probe the brain and tell with a reasonable certainty if someone is lying. It may be possible to produce a machine in the future that can analyze the mind well enough that it will be possible to tell what words a person is thinking. If that's the case, would it be legal to require the accused to be subject to such a machine to reveal their password? If that is the case, will the government need the accused's testimony at all?
Further along that line of thinking, it may become possible to change what a person is thinking by suppressing or exciting different regions within the brain. At that point, is punishment still legal if the government can simply change what a perpetrator thinks? For example, changing a con artist's mind so they no longer think of using their persuasive skills to con people. On the surface, isn't that what prison and the penal code is about anyway?
I believe we need to concern ourselves greatly about this, because we are now on the cusp of technologies that will make building brain-computer interfaces not only possible, but possibly simple. This will be an amazing boon to the elderly, others who have degenerative problems, and probably everyone else. At that point however, where does an individual's mind stop, and legally-accessible file storage begin?
I believe the slippery slope of both these arguments starts at this point, deciding where the demarcation of self-incrimination is. What we decide as a society over this argument will have a significant affect on how our ultimate future will go.