OK, I experimented a bit this afternoon. The black oxide will be removed by the Sta-Brite flux and you can solder to it. However, as you found, it will take much more heat that 30W pencil iron can supply. I only had an oversized example but my 280 watt Weller gun didn't come close to being enough. I have an ancient American Beauty 200 watt industrial iron and that was pretty good, but I ended up using my propane torch - very carefully and at a good distance to avoid overheating the joint - to get good flow. I would guess a very heavy gun or ancient industrial iron would be sufficient for 3/32 wire and the proper sized roll pin. How you would ever convince yourself you got good joint in the places you can't see, I have no idea, and I also have no idea how you could get all the very corrosive flux out of the joint. With this much surface area, there would be no issue with the strength of the solder itself, as long as it didn't later corrode.
Sorry, but your second try with the bare roll pin isn't really good enough. The solder has to flow like water into the joint, and this is just sort of blobbed on there.
*If it was me*, I would forget about soldering it and use a carbon pushrod assembly with the stub pushrod coming out of the wing. I assume you don't have that at hand, so a second choice would be a lap joint with maybe 1 1/2" overlap. Clean everything, apply flux everywhere, wrap the ends of the lap joint to hold it together but leave the middle section open. Solder the music wire parts together using a much hotter iron until the joint is solid. Then remove the wrapping wire, clean some heavy bare copper wire (like you would wrap leadouts with), clean the soldered pushrods with a wire brush and solder, flux all over the outside, close-wrap the joint from end to end, then solder that until every bit of the wire is flowed cleanly with water-thin solder on all sides. Clean the finished joint with first water, then polish it with the tiny Dremel wire brush from end to end. Remove anything that looks like a black spot (which is probably corrosion).
This should be good enough. For the future use a single-piece music wire pushrod, or a graphite arrowshaft pushrod assembly.
Brett