I've read lots of discussions of this question in a variety of contexts.

IMHO the most important thing to remember is that the human beings at the gate are the final authority.
They can follow guidelines, or not.
They can choose to read your letter, or not.

If you cannot convince those humans beings to allow you to board with the instrument, the instrument does not go on board with you.
There is no court of appeal.

If you raise a fuss, they will call security and have to removed from the premises (at best).
You are your instrument will miss the flight.