It has been my experience that riding jumpseat with American carriers is not a particular problem if you're ATC. You need, however, to clear this prior to departure with ops and the captain. Once OK you are expected to remain up front for the duration (except for a pee or meal break). What is generally NOT possible, even as ATC, is to request a flight deck visit during the flight if you originally boarded as a passenger. I have jumpseated with NW, AA, DL, WN, HP, and a dozen other U.S. airlines -most of them now sadly long gone!