If you search for a one-way ticket the flight just does not show up. You can find that flight if you search for a return ticket, which costs 42000 avios + 330 € for two random dates I picked.

I assume you can book the one way by phone, but as you can see I don't believe it will be better than with BA. I think that AA/US redemptions in general are better booked through BA (you also get a more flexible ticket).