I literally did this a couple of weeks ago when Ripley decided to recreate Alien 3 and dive to her death from the shelf that I had temporarily placed her on while cleaning and sorting my shelves. Her leg snapped off at the hip joint, just like that.
What I did was I slowly, with low torque, drilled holes into the center of both sides of the break (the snapped pin on the leg, and where it connected to on the hip) with, in my case, the second smallest drill bit I had. You need to be careful, and drill the hole deep enough for a good hold, but not drill it too wide, as the plastic is brittle and could split or crack or splinter easily, and then you have new problems. If you need to widen the hole gently wiggle the drill around and angle it out a bit to shave a bit more off that way, until the hole is just big enough to fit whatever you want to use as a pin in it. Now I didn't have a rod of any kind at hand, so I cut off the head of the thinnest screw I could find and used that as a pin.
Now because of the brittle nature of the plastic if you are using a screw I don't recommend using a smaller hole and trying to screw it in by force, because there is a good chance that the pressure of trying to do so will split the plastic (though remember, I was doing this on a small and thin female body, so you may have more leeway with something bigger like the Hulk, so follow your instincts). Instead try to have your hole as close to the size of the screw/rod you are using as possible. Keep testing and going bit by bit until you have a snug fit in both holes. Don't try to do just one side at a time, get the holes right on both sides and make sure the rod fits properly on both ends before you assemble anything permanently. Also, remember that you don't have to bring both sides of the break back flush together, it is okay if the rod is showing between them a little, just as long as it goes in plenty deep enough to both sides of the break to hold.
Now if you are sure the holes are deep and wide enough, and the rod fits in well to both sides, then put some glue in the hole or on part of the rod and put it in place in the hole in the broken joint, then set it aside and let it set in place. Later come back and test that it doesn't move and is firmly in place, and then do the same with the other end of the rod, gluing it into the other hole and letting it set. All going well, not only will it be back to being fully functional, you now have created probably the strongest joint on your whole figure. It can be fiddly and time consuming, but as long as you don't rush it and get too impatient, it works.
Note that if the body you are working on isn't quite as brittle as the one described above, but is a bit softer and more pliable, then a cut off screw as a rod is more likely to grab, which is a plus in its favour, but even then, I'd still use glue on both ends just to make sure it stays put long term.
I'd show a pic of my Ripley fix, but she's back in the display case, and it a bit of a pain to undress and redress, but if I can do it with my low level skills pretty much anyone who can use a drill should be able to.