Thanks for all the quick replies everyone
I think I figured that problem solvers kit out. Before, I was thinking that it would somehow replace the broken stud only. Now I realize that the screw is a replacement for the screw for the screw that keeps the cantilever on the boss. The replacement screw threads onto the remaining thread (which thankfully is still good in my case, and I think should be the same threading as the replacement screw) and the aluminum sleeve acts as the broken off stud (with the actual cantilever brake slipped onto the sleeve). Since the screw is now deeper into the boss, if it happens again the whole thing might get pulled off, but it definitely beats brazing a new one on!
And I thought I was going to have fun with torches and crazy drill bits! But then I would have a bigger problem when my house burns down...
Thanks again everyone, cheers