I wonder if somebody could make an open hardware version of the Leap Motion with this technique (though I'm not sure how accurate/repeatable the sensing is - Leap Motion could detect with an accuracy of < 0.7mm)

That's a great thought. The challenge is that Wi-Fi sensing (CSI) is measuring multipath changes across a few meters, not direct motion in a small volume like Leap Motion. I think its accuracy is measured in centimeters, not sub-millimeters.