Eye materials in Big Buck Bunny

I’m studying the eyes in BBB characters, and there is something I don’t understand:
The eyes use 4 material indices, but some indices are not assigned to any faces (for example: indices 2, 3 and 4 in the chinchilla).
But these material indices seem to be visible in the 3D window when the eye is in object mode (what is very useful, because you can see the pupil for aligning the eye).
I’d like to reproduce this, but for all I’ve tried, I don’t know how to do it. Can anyone explain me the BBB eye material?
Thanks in advance.