I often fly out of Nice to the US using AA in connection in Madrid or London. Well aware of this problem, I always try to book when connecting in London with BA flight number.
The trouble is, when I go through Madrid, that on IB flight number, the mileage credit on european flights is .6 of the actual mileage (even used to be .3). So I do take care to book Nice to Madrid on AA codeshare . This only happens when I can't avoid Madrid to chose London.

Up to now, I have never been able to pre-reserve my seat on IB with AA number. Even if, I was ready to pay.
In the past, AA reservation agent on the phone did asigned me a seat, wich assignement was totaly disapeared the day of departure (IB erases or ignores it).

Yesterday, Sept 28th 2012, I went to the airport IB counter: IB agents did try hard. The fact is, AA seats do are "bought" but passenger names are unknowed to them, "you are in a pack we can't see". No need to even think of my oneworld status...
So it is useless to look for a PNR that doesn't exists. Looks like Iberia computers retreive the names of the AA passengers only the day of departure, and only at the airport checkin counters computers. Whatever fare O to Y or even business.

So when I do board that IB flight, I always feel lucky to even be there :-)