My mother plays hammered dulcimer and what she did was she wrote the TSA ahead of time about it and they sent her a letter with the law. When she got to the airport she explained to them that she was a professional with an extremely valuble instrument. When the people refused to let her gate check she showed them the letter and then when they were unhelpful she asked them for their names so she had sufficient details for the lawsuit if anything got broken. It worked like a charm. That method won't work as well if you are a poor looking though so dress nice and be very polite.