You would have to heat the frame tube to over 1000 degrees F to get the filler metal to flow into the joint. This heat will flow outwards through the tubing damaging the paint. You could get some scrap painted metal and try it to get a sense of what happens. If you could engineer a way to capture this waste heat (say with a liquid nitrogen jacket), you just may pull it off. But then you would have waste much time and materials to verify that it would work.

You are looking at a temp fix now (which may actually last for a while) until you decide that you need a new paint job. You could have just the top tube painted - or maybe find someone who could blend in a touch-up for the area that you mess up while brazing.