Dave.
All I use is a plumbers propane torch, on original barrels I do not use a mask I just clean off the part I want to solder For soldering I use a resin flux and wire flux solder,these are the best to use on damascus there are other fluxes and solders which are very corrosive.You mention the barrel lug which I think you mean the lug the hole for the ramrod and the wedge,you also mentioned brazed ribs this was common on Belgium and Continental makers.
Back to the lug first clean off the rust and old lumps of solder from both the lug and the hole in between the barrels in which the lug will fit leaving a thin film of solder which will help to bond the joint. If the top and bottom ribs had been only soldered instead of brazed then they would have to be wired and pinned above and below the lug. Next tin the lug and place it in the hole and flux ,apply heat on the lug and when the solder runs force the the lug into the hole using a piece of wood , when the solder has froze trim away the excess that has overflowed ,that should be the finished job .I have now been shooting and restoring muzzle loaders for the past 60 years and that is my method no doubt there will be more modern ways
Feltwad