Hi Mike! My husband and I watched "The Greatest Show on Earth" last night. We did come in about 20 minutes late, but otherwise spent the entire time looking for old clowns! We have a (very) rough theory.
There are SO many circus folks in the show. Yet Arthur got billing. I think union rules (Screen Actors Guild?) mandate that if you have a speaking part, you get a credit in the film. So maybe, Arthur speaks? I think we have a candidate. After the train wreck, Holly, (the Betty Hutton character) starts issuing orders while Charleton Heston is hurt. She turns to Emmett Kelly and issues some order and then turns to an older man, next to Kelly and not in makeup, and tells him something to do. He says "Are you crazy?" and off they go. I think the "Are you crazy" man might be Arthur. He's next to Kelly, he looks older (60-70s) and he speaks. That was our best guess.
I found a contact for the Paramount Film Library. It's here: http://www.paramount.com/paramount.phphttp://www.paramount.com/paramount.php Hope you can get to that. It's sure worth an email question!