Just wondering out loud, here...
If you want to shoot black and white and color images of the same subject with one single exposure, I wonder if it would be best to do it with something like the three-strip camera used in the original Technicolor process. The final print could be made by dye-transfer/imbibation.
You could even use the same camera setup but with only two strips of film, one traditional tri-pack color and the other black and white. But, if you did use three negatives you could combine them any way you want to produce your desired effect. If you wanted to print them with the traditional CMYK matrices, no adjustments need to be made but, if you wanted black and white you simply print only the matrices that are needed to produce your desired effect.
(Could it not be done by simply printing the blue matrix as black?)