Fraser-Lu mentions it, but does not give it a name. She calls it "inlay" (see pp. 148-49 & figs. 142 and 143). She essentially quotes a passage of Bell, "Iron and Steel Work in Burma" (1907), which is the only reference I have found, other than Lu, that discusses it.
Nick - overlay is a good word for it (why didn't I think of that?
). It is not exactly inlaid, because it is not forced into inscribed patterns, but rather applied to a broader roughened area on the metal surface.