I'm not sure if this is possible, but I figure it's worth asking. Is it possible to create a shader that will only render at points where it intersects with other geometry? See the image below. Basically, the grey box has a standard diffuse shader whereas the other box has this "special shader" that make it transparent except for the points where it intersects another mesh. Imagine that the black lines don't exist on the special shader, resulting in simply a grey box with patches of green.