For the time being, I'm solving the problem by duplicating my Form sphere, placing it above the .png layer and fading out the opacity as the camera approaches. Is it similar to CC Cylinder in that it's just not real 3D?
"The secret to creativity is knowing how to hide your sources." --Albert Einstein
It's partially real 3D :-D. It works a lot like other 3D effects in AE. Particular helps with the Obsfucation property. Unfortunately Form doesn't have that. You may try with the Visibility properties to see if that provides a solution. Otherwise, use an additional instance of Form with the other 3D layer used as a track matte.