Originally Posted by
schreibsman
Once the flight is closed for check-in, it will be even harder to convince an agent to open the flight and do this for you, and indeed it likely will take a supervisor to be able to do this. They begin working on weight & balance and all sorts of other things during that time, and fussing with loading/unloading pax can make things more complicated.
I should have mentioned that my experience being un-checked in by AA was about ten hours before departure.
Incidentally, I have had BA un-check me in a number of times as well (all more than an hour before departure).