[jd callow]

This thread WAS about ortho film. Some had suggested filtration to render an 'ortho look' with pan film.

Subtractive?

Yes.
Apply filter(s) that are a minus type. When you filter for B/W you are not adding colour but subtracting (neutralizing/removing to whatever degree) the colour. A minus yellow (blue), minus magenta (green) or Minus red filter (cyan) or the combination of these would tend to get you close to ortho on a pan film.