He was on a poster and in a hidden Arkham City Story in, well, Arkham City.
Soooo... that's something.
Oh I know about his references and his Story. I'm just hoping he actually makes a physical appearance sometime.
Also, I want Rocksteady (if they do the next-gen sequel) to put in a reference to Condiment King from The Animated Series haha. He was a great joke villain. "Come Batman, let's see if you can cut the mustard!"