If it had been me, I would have said it was a mistake not to move more forcefully pre 9-11 to declare Al Quida a clear and present danger, and even with the obvious teeth knashing of the usual suspects, have declared to the Taliban, that the perp needed to be given up, and the terror camps closed, or the gloves would come off. I should have been willing to take the heat that most would have condemned me as a trigger happy cowboy. I would have said that it was a mistake that I didn't let the essential Bush be Bush earlier. But it is better late than never, and I have learned from that mistake, and never again will I make that mistake, even if it costs me re-election. The matter is simply too important for our nation for me to do anything other than exercise my best judgment as to what is right, whatever the political consequences.
That is what I would have said.
He handled the question about his biggest mistake since 911 well. Let history record it, in essence, is what he said. That leaves no room for the media to immediately pounce on any other answer and leaves open the possibility that nothing of historical significance will ever be widely pursued or accepted with any lasting quality.
Your idea of mocking those who've heaped criticism upon him in that manner has some humorous quality, but doesn't answer the question about post-911.