IANAL, but setting up such a system could not be illegal. At the time you set it up, you cannot know who will be forcing you to unlock. It could be a member of an Armenian gang.
What happens when you are required to unlock is less clear, IMHO.
“Unlike disclosing passcodes, you are not compelled to speak or say what’s ‘in your mind’ to law enforcement,” Gidari said. “ ‘Put your finger here’ is not testimonial or self-incriminating.”
But are you required to warn them about "bad" fingers?
And you might not have told you girlfriend which of her finger, will start the self-destruct.
And even if you are required to warn them, if you _do_ believe that your rights are being violated, now they have to prosecute you, and you can argue they had no right in the first place.