I thought the Key was meant to actually be Brainiac. They were a bit vague about it on the mini-commentary
I think they said something like that too, But I also remember them saying he was going to be some sort of conduit. It seems like they had two directions that they ultimately didn't go with.
You say that as if they wouldn't do something like that, but we all know they would.
At the time I think they were pretty adamant to not have Batman's supporting characters show up, they wanted it strictly be a JL show. It wasn't until JLU that they gave a bunch of fanboy nods.