With good optics and fast tracking, I don't see why not. Even if your video is lower-resolution, you could use a video feed for movement tracking to direct a high-res still camera with decent optics and have a high-resolution capture of the face and eyes, maybe enough to distinguish iris details. 6 metres doesn't look hard for my off-the-shelf optical-zoom camera.

If you are rich and can afford custom optics and a large-ish array of sensors (think of Kepler's camera back, more rectangular and much smaller), you could capture different sensors independently to get different sections of a larger panorama separately, identify which ones hold high-enough res irises and pipe them through your recognition plumbing. Knowing all irises are within a certain range of heights make the problem a lot simpler WRT positioning the camera, the optics and the sensor array.