I had no problems with harmonicas in my hand luggage to the States a year ago ( I have a box with 9 of them, plus 2 jew's harps and 2 capos). They'll ask you to open the bag to have a look, once they know what they are, they'll let you through. You might have to play a few notes to prove it, but don't play too well, or they'll think you're professional (grin).

Splott Man