Sadly it's just a stage transition. You can see it in the Wonder Woman vs Harley match, here is the link
http://www.youtube.com/watch?v=fmcDR0QqU5U
ThisDon't assume he's playable. That could easily be an interactive stage element for a special Arkham Asylum stage. Just look at the characters and environment, it's all straight from those games. Also, this could simply be a part of the story mode. But hey, if Croc IS playable, I wouldn't argue.