Yes - drilling small holes on circular surfaces is difficult.
Do you have a small dremel-type drill with some disc cutters? If so, a light cut across the top of the tube would be easy to do accurately and would leave a small hole. If the cut was too deep it might weaken the brass tube, but for a static model that would be fine. The solder would flow into the cut and hide it completely. You could even do it with a triangular needle file.
If the cut did not go completely through it would still help in centring the drill. The nice thing about drilling holes is that you then have 1/8" excess tolerance that you can cut the vertical wires to - they will slide up and down in the tube...
You can also cut tubes precisely to shape, then join them temporarily with a bit of mains copper wire inside, making very neat joints. See pictures 5, 6 and 7 on this page -