Your special login shell would not allow you to do anything except provide your secret, after which you are simply disconnected without having the opportunity t

Your special login shell would not allow you to do anything except provide your secret, after which you are simply disconnected without having the opportunity to do anything else. A different process not under your control would mount the volume. At no point would you get to execute arbitrary commands or open any files. You would only have to be trusted not to hold your secret for ransom.