If you can grip the top/bottom of the boss with some needle nosed pliers you may be able to unscrew the bolt , then with a rivnut tool you should be able to pull the rivnut tight against the frame tubing , you may be able to put a drop of very runny glue (I'm thinking super glue or permanent loctite or laminating epoxy resin) on the join First and let capillary action draw it where needed, be carefull not to let the glue get on the female threads. If you need to drill out the old boss and put a new one in its place it will have no matching paint on it. good luck