Mutli Texture Transperecny using a worldspace controller is it possible?

(Hey guys I was wondering if this was already done or created by chance). I’m hoping to create a rig with multiple layers. First layer would be skin, 2nd layer would be muscle, 3rd layer would be bone. Now its obvious that the transperecny is using the UV values of 1-0. (1 being white and black being 0). However I was wondering if there was any way possible to create a physical object such as a nurbs sphere with a world space influence of 1 in the center with a gradiation going out. So if I dragged the sphere over the body it would show the skin then muscles then stop at the bone mesh. An example of this would be
http://www.youtube.com/watch?v=zM3psSxWTYM at 39 seconds in the sphere has influence over the blend-shape.

Hopefully you guys know of anything to point me into the right direction!

Thanks in advance