Contrast masks were often used for high-end colour carbon and carbro. The masks were "tuned" to correct for colour poisoning ofthe specific pigments employed. The contrast, not just density is the critical part. There is a complete description in one of Luis Nadeau's book.

If you are going the digital route, although software can create 4-colour separations, they are usually geared to screened offset-printing, which doesn't always translate well to carbon/carbo processes unless you are using the same pigments, and can achieve the correct contrast.

Been there, done that, and lived to tell