Implant something into muscle memory that can be used as an encryption key. (Muscle memory has to be retained, right?)

I'm not sure exactly how to go about this. But for example, I've heard of people who say they don't know their passwords for certain sites, but their fingers know how to type them. So leave a message that says "go to this website, use the DOM editor to change the password box to a text box, and try to log in. Use the password that your fingers know as the key to decrypt this other message".

This particular version probably wouldn't work, but something along those lines might.