I saw that the Canvas element support rendering drop shadows, and by what it seems this includes images with transparency too.
So maybe could be possible to create a behavior that applies drop shadows to objects with parameters Offset X, Offset Y, Blur and Color.
I'm not diving into plugin development yet, so if it's feasible and anyone wants to give it a try here's my suggestion.
Probably would be harder to make it work with WebGL, but even if it's Canvas 2D only should be useful as well.