I imagine this attack is more difficult than is assumed.

The device itself is likely tamper resistant, has built in DRM and will have a high fidelity camera + signal processor. So you will need a system to write a 200MP image on the surface the size of an eye ball.

Also, since the light passes through a lens in your eye, there are probably optical tricks that can validate the surface is curved like a retina and has been distorted by a lens.

Oh yeah. And the vascularization of the retina means that you can view heart rate pulses. So your image will need to pulsate too.

One could also imagine marrying this with a stereoscopic camera to image detect that a real face is. It could even ask you to smile or frown to validate it's a real face.

Lots of ways to harden this technology to the point where even intelligence agencies would struggle to defeat it.

How about contact lenses similar to those like for halloween but just don't look creepy? I'm pretty sure you can already buy those and they should generate different hash. Face probably can also modify with makeup/mask/beard etc - after all face recognition is not perfect on iPhone any many times cannot automatically detect faces.