That sounds meaningless to me. Polygons are "flat on the ground",
therefore they don't have a shadow.
If you wanted to have the polygon "floating in the air" you'd have to
recalculate the lat/lon points by adjusting the Earth radius to be
Earth radius + altitude.
But if you wanted to do that, sure there is a way. (There is a way for
everything!). You could use another gray polygon to represent the
shadow, but again, you'd have to adjust the lat/lon points so that the
main polygon appears to project a shadow proportional to its
elevation.
--
Marcelo - http://maps.forum.nu
--