I am a frame builder. I have (stupidly) drilled holes in thin gauge top tubes and even brazed a reinforcement washer around it. After a while, it cracked. I am glad to hear that you are doing something, because once it cracks, you are looking at a top tube replacement/ say goodbye to your chrome. I have pulled out and filled in my share if misplaced waterbottle cage receiving nuts. Do not plan to fill in entirely with brass. Brass does not work well filling a 1/4" hole. I take a drywall screw, cut the threads off. put the screw cap in the hole and braze it in. Next, you just grind/ file and then sand it down.