Book them with no AA number (i.e. not logged in just a regular Jo(anne) Schmo site user) and then you have to call up and ask AA to manually insert the BA number for you - whilst you're there, ask them for the BA/Amadeus reference too for easy retrieval on ba.com. Or, wait for ~24hrs then try using the AA booking reference in MMB on ba.com and from there, insert the BA number yourself. This seems to work these days (the 24hrs is to allow the systems to sync up).