AA can book a RTW without any international AA segments BUT the ticketing & fare desk dept will refuse to price it and thus you cannot issue the ticket.

If you can, try and route a segment that has both AA and BA flights on it ie LAX-LHR. Book that segment as an AA flight, but then cancel it and change it to BA on the day of travel.

Alternatively, you may book anyway you like but always include an international (does not incl US-Canada) segment on AA. You may then reissue the ticket later with either BA or CX and route your true routing.

Alternatively, you may try and issue the ticket with either BA or CX in WAW.