This happens a lot Peter!
It is most often bits of brazing material that end up marooned in there after brazing up...
It IS surprising that there is not a vent hole anywhere.. not inside head tube? Have you removed the headset to check there?
When brazing, most especially with brass, the heat inside builds up so much that a vent hole allows steam/gas to escape there at the vent rather than in the joint you are brazing ("pin hole.")
Anyway, if there is absolutely no access to the interior of the top tube, I think the head tube is the best place for hole placement.. That is because it is a thick tube relatively speaking and can suffer more abuse (!)
If you can use a long sharp center punch, you may be able to drill a small hole at the angle required. You may need a bit extender...
Then, if and when you get penetration (no guffaws from the peanut gallery, please) pour inside the top tube maybe 10 table spoons of Linseed oil from your friendly local art supply shop. (LASS) Then turn and twist the frame around to distribute the oil uniformly around inside the tube, drain it out again and let it sit for a week or two undisturbed.
What will happen is that the Linseed oil will "set up" trapping the particles and coating the inside of the tube against future rust and corrosion.
Dale Brown