For positives and assuming a reversal process, a staining developer as first developer would definitely not be a good idea since you'd be left with the stain after the reversal. So the question right off the bat is whether these assumptions are correct: are you indeed doing a chemical reversal process, and have you indeed been using 510 pyro for the first developer as well?
Or are you referring to 'positives' in the sense of a very thin negative image that shows up as a positive against a dark background, like a tintype etc.?
I honestly don't think it would matter all that much, but if this is going to be a black/dark silver image on top of a gold background, you'd want something that yields sufficient density. I'm thinking along the lines of Dektol or ID62; something fairly clean-working that will build lots of density. Btw, I've done pretty effective (albeit somewhat 'blingy') 'orotones' by transferring carbon prints onto glass and then backing the glass with a mica faux-gold pigment.
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This is a test print made as outlined above. Not exactly what you're looking for since it's not a silver image, but black pigment in gelatin. The end result looks similar though.