It's absolutely possible, from a systems perspective, to offload a passenger who is already checked in. AA may have a strict policy about this and train their staff not to do this unless circumstances require it.

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.