According to my last flights, for the first leg I can get it at checkin at the airport but for later connections I need to ask at the departing airport (i.e. gate) but not necessary the departing gate, any gate seems not busy can (and done for me ) do it.
I heard two version: One said bulkhead seat do not release until check-in start in the airport, while another version is 24 hours before flight. They block bulkhead for passengers travelling with childern.
BTW: Have you try to talk with AA? If they block bulkhead for childern, I think with your GF's situration, they may be able to do something...IMHO.
Good Luck to your GF and wish she not only get the bulkhead but OP-UG!