Based on the direction of the sun, assuming that when you first open the screen to create a new layout you are facing north, which would only make sence, could you not create the shadows (i.e. drawing the shadows onto your image) while creating your texture maps?
It would require a bit of artistic ability, but as long as you keep all your shadows on all the newly created assets at say 5:00pm sun in the west, I don't see why it would not work.