Yes, you could nickel plate your hammers to hide the brazed repair joint. But they will look like they are nickel plated, not steel. I think I'd rather look at a braze repair than nickel plated hammers.
Another option might be to use a product like Birchwood Casey Brass Black to make the brazed joint less obtrusive. I used some years ago to darken the brass furniture on a muzzleloader, and it held up pretty well. Of course, you could also attempt to completely remove the braze repair, and then try to have the spur TIG welded back on. I kinda doubt if you are going to want to melt that braze joint, just to determine the approximate melting temperature.
