I haven’t looked at the code that handles that for a long time so I can’t say for sure, but I don’t think light intensity is actually passed on because it’s nothing more than a multiplier for the colour.
I haven’t actually tried to do anything with it, but the aforementioned setting to make my model output a true lighting buffer is the best way I can think of to accomplish this. You can write whatever rendering logic you want in a post process utilizing that data, to lerp between the basecolour and shadowcolour buffers.
I haven’t experimented with it, but if I was going to play with lumen and stylization that is what I’d try first.