It's not uncommon for framebuilders to use silver for brake bosses, as long as you have a good miter with no visible gaps, and get good penetration all around. Brass/bronze is more forgiving.

Since cantilever bosses are relatively cheap, do a practice run and test the strength. That's what I do when attempting something new.

Example: when I was working on a new decaleur design, I did a destructive test on 3:


You can see a small crack in the filler on the right side, but it had to be bent 90 degrees to the side to get that way. I called this one "strong enough"