I went to MOMA today to see the Atget show, and afterwards walked through the rest of the photos galleries. I saw a one-light Avedon portrait from 1965, and leaning in to examine the catch-lights in the eyes -- it looks like a plain old strobe and umbrella -- I think I recognized Avedon's silhouette at the edge of the catchlight. I remember seeing him stand beside the camera in that Laura Wilson picture of him shooting in the West: http://www.variety.com/article/VR1117983002?refCatId=2870 He may have been doing that here.
You can see the full portrait of Lady Eliot here: http://postimage.org/image/213wdsbms/