I had an idea for modeling a new eye entirely that would include a few more surfaces and layers.
Specifically, an inner and outer iris surface and an inner and outer pupil surface, an inner surface of the back of the eye to imitate a retina, and an actual lens.

The inner surfaces of the iris and pupil could be separate objects, not physically attached to anything and could have trans maps applied to them to eliminate that unrealistic hard edge look.
Similarly, the iris and pupil could be single objects, but with higher resolution to be able to be dilated without texture stretching.
I'm also thinking about maybe two layers of iris, to give more depth and to allow for more effects.
The inside of a pupil really isn't solid at all, obviously, and the only reason it appears black is because the retina absorbs all light... and I think light can't pass back through the lens the opposite direction due to refraction.

The inner eye surface would be facing inward, against the back of the sclera, and would be there so it could be textured black and/or made 0% diffuse to simulate the black pupil, in the event the inner pupil were made completely transparent and someone wanted to get the effect without using raytracing, while also avoiding specular highlights on the pupil itself.
Or it could all be used with correct refraction values, and the lens would help with that effect, by refracting the light and making the pupil appear dark.
That's what was in that render I posted many pages back when the eye discussion began. In that render, the pupil is completely transparent, but the lens behind it and the refraction I used made it appear black, like it would for real.