This is an unfortunate effect of the removal of YY fares
AA cannot do much other than ask EY and QR if it will sell the ticket. Even the most expensive QR and EY fares have sales restrictions that do not permit sale of ticket by AA
In most cases I have come acronss, airlines will refuse to sell an infant ticket other than in conjunction with a ticket it has sold. I have managed to get QF and CX to do it before, but generally seems to get a no when asked
If QR and EY stick with what seems to be their norms, you may have an issue getting an infant without seat ticket