When 753 first came on the market, Reynolds said no braze-ons of any type were acceptable. But people being what they are, over time 753 frames with bottle mounts, lever bosses, cable guides, and (heaven forfend) even front derailleur mounts were made with no undue problems. But all those can be attached with low-temperature silver, while I'd be leery of anything but brass for brake bosses. At best, the higher temperature of brass brazing would reverse the heat treatment 753 tubes get and leave you with plain 531. At worst, who knows? If it were my bike, I'd get a different, non-753 fork for the brake bosses.