I can't really tell what the fitting or the problem is from the photo. It looks as if it may be in a solar heater. The solder looks to be gobbed on the outside of the pipe cap, not drawn into the joint. The drop tube that was capped was probably silver soldered to the main tube. Is it the cap that is leaking or the down tube joint?
Sometimes things get so crudded up, it is beneficial to just replace a section of pipe and get back to some native pipe in good shape. The rust and dirt pattern almost look like the main pipe or down tube joint is leaking. The joint needs to be hot enough to draw solder into the joint, not lay on the outside. Have you ever removed the cap completely? If you can remove the cap and you can see where either the cap or the pipe is not fully tinned. If they are completely tinned and properly heated, they will seal as long as there is ABSOLUTELY no water making steam in the joint area. If you can't remove the cap, there is a chance it was silver soldered, a totally different process that requires a totally different solution.
It looks as if the clearances are extremely tight. I would try to get the incoming large pipe cut loose, enlarge the entry hole at the frame to see about getting a bit of wiggle room at the top of the frame. If you could get the main tube cleaned up and cut, you could bring in a fresh piece of pipe with a coupling and eliminate the by-passed tube. I don't think you will ever get enough clearance to use a tubing cutter, think jig saw or Dremel. Once soldered in, waterproofing the enlarged hole in the frame should be fairly easy.