Nope, not over the buses.
Regarding the entire premise of this thread, by definition, once the state of emergency was declared, city and state officials, meaning Nagin and Blanco and their designees, had the authority to commandeer any property, public or private, to use in aiding in the response to the emergency. They could confiscate your house or car or the entire contents of every Walmart in a 100 mile area and use it as they saw fit, so they had the authority, under law and custom, to do what was necessary with the buses.
Nagin had the authority and failed to exercise it. PERIOD. It's not even close to being arguable. He may not have realized it, but that only proves his incompetence to hold the job. he may have been God's gift to New Orleans in terms of working out the day to day issues, like garbage pick up and cleaning up corrupt bureaucracies, but he was completely unprepared to exercise executive authority in a crisis.
Regarding the point about lacking "first responders" to man the buses, he had the option of conscripting just about anyone to the task, including the 8,000 national guardsmen that the governor refused to deploy. Or he could have offered the bus driver jobs to the cops who were wanting to bug out because it was just too tough to do their jobs.
And before anyone gets on their high horse about the "brave first responders" being above criticism, I spent several years as an EMT in New York. Been there, done that. There are good guys (the ones that stayed on the job) and bad guys (the ones that bugged out). The ones that bugged out ought to be imprisoned for many many years for manslaughter under the "depraved indifference" principle.