Unfortunately this does not work. I tried both "Brush" and "Tree" Layer without success. I will be doing my own testing with this issue as well.
Okay so it appears that setting (Basic) Art: Layer field from Grass to Brush at least increased the proportion of dynamic grass that generates with shadows. However, there are still patches without shadows.
From running multiple generations, it definitely feels like more grass has shadows than before.
Using the setting Tree gives about the same proportion of shadows as Brush.
So there are more shadows which is a definite improvement, though still not optimal.