Thanks for the info Himeno, but I'm not sure your reading of the rules is correct. The xONEx fare rule reads:
NOTE: For flights where First or Business Class is not offered or available, passengers may travel in a lower Class, in the applicable booking code for that lower Class. The fare for the highest Class used applies, without compensation.
Exception - for services within the USA where no Business Class is offered, Business Class passengers may book and travel in First Class (Booking Code "A"), subject to availability. This provision does not apply on any flight where Business Class exists but is unavailable for booking
The rule doesn't actually restrict this to AA flights, just flights within the USA where no Business class is offered.
Do you (or anyone else) have first hand experience of how this would be ticketed using QF codeshares for flights within USA?