Against all odds, with nothing but a few humble tools, and nothing but my willpower and ingenuity to guide me, I have overcome all obstacles, conquered all technical difficulties, brushed shoulders with failure itself to finally fix the joint that no man has fixed before. As I lifted up my numb fingers to carve away the final portion of hardened epoxy putty, I have come face to face with God himself. This is a story....
...that would have been nice if it were true, but is, in fact complete bogus. Turns out that the solution to the broken joint isn't nearly as complicated as I imagined.
When I finally started fixing the joint, I rummaged through a few of my old gunpla boxes which were largely abandoned and dutifully gathering dust. There inside one of the box was some spare polycaps that I kept as spare parts in case something like this hinge need to be replaced. So basically, all I had to do is cut out the front portion of the polycap for the wing joint to attach to, trim the front corners so the wings can pivot more easily and then just sheer off enough of it from the side so that it can be fitted into the wing's joints.
This is the result
The polycap's peg was a bit too small for the hole on the backpack, so I just used the age old method of painting a layer of black paint to increase the peg's thickness.
With the wing attached to the modified polycap, the result is quite good and other than the color, which I plan to correct later, really have no blatant flaws. While fixing this hinge system, I did at one point consider replacing the whole thing with one single ball joint attached to a peg, but since I have neither the material nor the time to gather them, this will have to wait.
So that's basically all I had to do. The joint works pretty well and I'm glad with the result, but to be honest, it was a bit too underwhelming since I really did expect an epic quest of re-constructing the broken hinge. After all said and done, one lesson has been learned and that is to always keep track of your inventory, or you could waste a lot of time trying to come up with and carry out solutions to problems that you could have easily fixed if you knew what those gunpla boxes of yours held within them!