OK, silver solder it. If, for some reason, it doesn't hold first time, silver solder on a backer plate, drill the screw hole through the backer, and gouge out the inletting to allow room for the backer plate. You can make the backer fairly thick for extra strength, too. The silver solder will work even if the guard is steel (except it will not blue).
How did you determine that the metal is nickle silver? Photos aren't always dependable, but I see some suspiciously iron looking discoloration in the photos.
