The pictorico transfer will work on revolver firewalls. Cut the art from the transfer sheet as small as possible. Try to register the section with the most coverage and tape it down. Burnish the largest area and then fold down and work your way around. You may have to go back and tweak a little...
I recently engraved NFA information on a plastic SIG firearm. Go very slow and try to guide the tool with minimum pressure. It is like driving down a wet red clay road but can be done. Try to find an obscure place to place the engraving.
I have had laser welding done with .006 iron binding wire and is undetectable and blues well. (Note) all laser fills will be harder than the surrounding metal so you’ll need to be a little more careful when cutting through the welded lines.
Red gold work hardens rapidly. When you are melting for an ingot it is important to use a clean (dedicated) crucible to avoid contaminants. Sometimes air cooling will work for annealling.
You can paint the area to be engraved with a silver Sharpie marker and the art will transfer or you can use Tom White's white transfer solution. A note of caution using these on certain painted surfaces. Sometimes the paint will come off with the excess solution when you clean it off.
If you will give me an idea on the type and size buckle you are looking for I will check my stock to see what I have left. You might check phase.com/tikkakoski and open the buckle folder to view some of the ones I have made.
Thanks, Tom