This is a good idea, but, I think the extra VRAM, that is needed, would ruin game play.
Back to your question at hand. You would have to take every sprite you plan on using shadows with and open then in Photoshop. Then make all the gradients in exact same size/shape as your sprite in an above layer. After you load all those new sprites in you would then simply use one set of sprites if NightTime is true or the other set if DayTime is true.
I was thinking there might be a way to use the Additive/Screen effects on a layer with a gradient but this wouldn't line up properly with each sprite. Who knows! >.<