Interestingly, about 50 minutes into the gate agents dealing with a hoard of passengers upset that in light of the cancellation they would not be able to fly until Sunday (this was a Thursday evening flight that got cancelled), they announced that AA was now running an extra flight on Friday AM and the passengers would be reaccomodated (AA 9203). Per Flightaware, the aircraft that was used was the one that came in the night before that was supposed to turn around and go back to DFW.

Still unclear why the late cancellation - if it was a crew timeout issue, that would have been apparent hours before they cancelled.