Thers is a little bit stupid suggestion. Truly, dont know is it possible or not with reshade, but needed to ask. How about a shader, that blur places onscreen that have same hue/color but different brightness/saturation? Idea is to emulate "soft shadows"
There are many structures where this also applies, same hue/saturation but different brightness. And on newer games, shadows are not evenly gray but colored by ambient color. Abandon the thought that we can access game specific things in postprocess by just analyzing the image.