This would be a handy function, as currently with human shaders, we have to use red overall colour, which contaminates the object. We could remedy this by using our own decay distance functions based on maps.
Take this object. The colour function is a custom map based on the inverted Transmission map, and the original textures, and some colour alterations. To my eyes, the control of the colour via the colour function makes for a better SSS effect. However, with the decay distance colour gray, it is noticeable in thin areas, where it becomes white.
If I was able to feed my transmission albedo map into the decay distance it would remedy this, as I'd be able to use the colour function with the original textures to have a more physically correct overlay.
With a customizable decay distance colour we could have more control over what colour intensities decay distance will have across our objects utilizing UV maps.
