I don't know all the different metal types used for inlays beyond what you mentioned. I do know there is a difference in the "quality" of parts manufactured/sold. I made a rookie mistake, I didn't have my drill bit aligned perfect and drilled a hole in my brass side plate off center. I ran over to Track Of The Wolf and purchased a replacement. The replacement plate was not nearly as heavy duty, much thinner, and not polished like my original. I was disappointed to say the least. Of course, once mounted and worked over you can't tell the difference just by looking.
Have you even use alternative things for inlay? Like, metal lapel pins, tie clasps, coins, and kilt pins?
I ask because that might expand the options quite a bit. I have been looking at some heritage inspired
options that are not commercially represented, but would not be out of place on a rifle.