I don't know... it may be one of those features that someone thought was a good idea, but never really implemented fully. You know how us programmers can be, sometimes.

Whenever I've dealt with manipulating the shadows separately, I've always just removed the white manually or by using a mask layer, so I don't know what elese there might be to try. Outside of Poser, I know that POV has a rendering option like that, where it will make the object invisible and just show it's shadow, but there has to be a visible object to receive the shadow. The shadow and partial shadow edges receive their color (even if it's white) from the object the shadow is on, so that object, at least, has to be in the image.