Spot plating does exist but whether it is adviseable for the described condition I don't have that kind of experience with it. Texas Plater Supply sells a kit that does electroplating with a brush, a chemical paste and a 6 volt DC power source which can be flashlite batteries. I have used their product at least 25 years, as recentlly as maybe two years ago. I have plated several complete guns and have touched up worn areas on many others.

If a chip has flaked off a gun I think the procedure would be to spot buff the area to blend the existing plating with the steel of the gun, degrease/clean the area and do the brush job. Without good prep, at best it would look like a nickel patch.

If you are comfortable with the work and/or willing to build up some experience on scrap it could work for you.