According to the book Avedon Fashion 1944-2000: The Definitive Collection, Richard Avedon was both the photographer and background model for this 1946 Bonwit Teller ad:
http://www.minimadmod60s.com/gallery2/main.php?g2_itemId=56503

...even using himself as a model, blurred in motion as he races into the background of an advertising photo of a dinner dress:


August 1946 Junior Bazaar Magazine


Jean Shrimpton
Avedon Fashion 1944-2000: The Definitive Collection
By Carol Squiers & Vince Aletti
(Abrams, 2009)

Richard Avedon made another rare appearance in this 1966 ad for Bernardo Sandals:



Susan Camp
Senior Editor & Archivist