That's a really good video showing how to use the shader mixer.  In this case I think you might want to try something like this:

http://www.daz3d.com/forums/viewthread/17295/

Apply the UberSurface shader (which now comes with DAZ) to all the surfaces in the object you want to cast a shadow but be invisible.  Then turn on the Phatom on/off switch.  It's easy to do this in the surface panel.  Just select the object and all it's surfaces, apply the ubersurface shader then scroll down and turn on Phatom.

Then add the item you want to be visible to the scene.  For it find the option so that it doesn't cast shadows.  Place it on top of the shadow casting item you setup.

If you do that you can have an invisible item casting a shadow.

Attached is a quick test.  I applied the UberSurface to all of V4 and turned on Phantom.  I then added the bear to the scene and set him not to render a shadow.  I added a plane to the scene and one distant light with raytraced shadows.  It works with shadow maps too, but I'd recommend raytracing for these kinds of effects.

 

 

-Matt