Or... just send an FA into the flight deck while the PNF is answering the call of nature, as is already procedure for many airlines
Not going to make an ounce of difference, I'm afraid.
My airline already does that, and given the average age, english language ability and education level, she (or sometimes he) will do anything I tell her to do, even as far as locking the door for me.
If I was determined to enact an act of disaster, a 45kg 19 year old girl would not stop me from me doing what I want to do.
Send in 2? Now it's a lot more difficult.