As someone who works on eye tracking for VR headsets, this is much easier said than done. Doing it well requires levels of accuracy, latency, and reliability that are not available in any eye tracker on the market today.
What about SMI already tracking pupils at 250Hz? https://www.youtube.com/watch?v=Qq09BTmjzRs That tech is still expensive and I don't know how accurate or reliable but if it is good enough for foveated rendering, I would have expected it to be good enough for focus as well. Care to elaborate as to why it wouldn't work, or is it just beyond your horizon of "today"?
It's not accurate or reliable enough (in addition to being very far from a consumer price point). Reliability in particular is difficult to assess and usually lacking in eye tracking systems. Also just because the cameras are running at 250 Hz doesn't mean the latency is 1/250 seconds or even close to that.
Only reason for lack of availability is size of the market. Tech is already here and you are probably holding it in your hand right now. Avago's will be more than happy to sell you tiny combo chip packing camera/DSP able to capture >10K frames/s, track at >5 meters/s and report position at over 1KHz.
As someone working in the industry, would you care to guess how many years away we are from being able to do this at an acceptable level? Is this something for the next gen of hardware or is it much further away?