Apart from Billy Bob Thornton, all of the actors in the credits of "A Hard Wayne's Gonna Fall" are names of other DC Universe characters: Daniel Cassidy (Blue Devil), Rita Farr (Elasti-Girl), and Garfield Logan (Beast Boy). All of these characters were, at least in some versions of their stories, involved with TV or movie-making.
When Clayface (disguised as Billy Bob Thornton) and the rest of the gang arrived at the premiere of A Hard Wayne's Gonna Fall a paparazzi yells "How's Angelina?". Thronton and Angelina Jolie were married from 2000 to 2003.
While watching the fictionalized movie about Thomas Wayne's life, "A Hard Wayne Must Fall," King Shark asks if Martha Wayne has had any lines yet. This is a very specific in-joke, as Martha Wayne had zero lines in Batman Begins (2005).