P: Desaturate Highlights and Shadows

In retouching there are many times when the highlights (or shadows) are contaminated by colours that should not be there. An example would be interior shots where different light sources were used (say daylight and tungsten), or where a wide angle lens has caused magenta/green shading on a white wall. It would be incredibly helpful to be able to select highlights/midtones/shadows and selectively adjust saturation. If, say, the split-toning slider could slide to the left to decrease saturation that would do the job!