but on my Linotronic 330 postscript driver, device options tab there is a setting for negative, which is currently set to false. My guess is that setting that to true would do what you need. Can you test it on a postscript laser printer to see if you get a negative?
I once printed a book to a file on disk from PageMaker Classic (version 4) and sent that for publication. That’s my only experience of postscript printing.