Yes, a six string bustier... (the back stoke on a jig could be interesting... )

A cod piece bass (for when you're REALLY upright and woody)

A bodhran mute (a net comes to mind)

A bayonet lug for my longneck banjo (for when it's really your "cut")

But my singular invention is the conversion of a neck brace harmonica holder into a sandwich dispenser (which goes very well with my beer hat - for when you get a really good seat at a great jam - one tube is actually a catheter, but I always forget which...)