Sorry if this is already answered somewhere, I couldn’t find anything helpul by "Search"ing.
Say, to make a grass blade, I create a polygon, then draw a “shape” b&w texture, and apply it as Alpha modifier. The blade is then shown as needed. I add a spot light, shadows on, and the shadow is the shape of polygon (square), not the “real” rendered shape of the grass blade.
I hope this is possible.