We know that Dan DiDio was thinking about Dick Grayson's death as the crowning touch in Infinite Crisis from early on, meaning from before the first issue in late 2005. Batman: Under the Hood appeared earlier that year, I believe.
I haven't read far enough in the Outsiders series to have seen the context of Nightwing's confrontation with Superman. The volumes and pages I have read, mostly pre-OYL, don't seem as out-of-character for Dick as some people complain. I think one theme of the series throughout is that Dick's idea for the Outsiders was one he couldn't really carry out, however much he wanted to after Donna Troy's death. Just trying it forced him to strain the friendships that he'd built up, thus straining his psyche.
In other words, we fans of Dick Grayson were supposed to be thinking, "You gave that job your best shot, but it's just not you." If Dick had truly been written out of character, then he wouldn't have had such trouble with that approach.
As with Brothers in Blood, the decision to spare Dick from being killed in the Crisis might have meant some quick rewriting afterwards.