Don, if I understand your post, are you saying that filters similar to a graduated neutral density filter exist in M&Y? If this is the case, it seems that a mechanism for adjustment and calibration would be easy enough to make in a home shop. Calibration would be the most difficult part to accomplish, but it would be a question of trial & error against a known standard which already exists from a diffusion enlarger with a color head.
Second, it sounds as if you are comparing the prints from a variable head with mixing chamber to a condensor enlarger, as the different results found between films developed with PMK and ABC. The diffusion head is a softer image, while the condensor gives the true clarity and sharpness which is inherent in a good negative with high acutance.