Nothing at all happened to the brass finding; it must have had some protective coating or oil on it that I didn't notice. I'll probably scour it and try again.
The fabric with bronzing powders worked a treat, though. The "gold" paint reacted only slightly, around the edges, but the "copper" paint took on so much patina that the painted streak almost disappears into the green background. (Duhh, maybe next time paint a piece of white or black fabric so you can really see what's happening.)