I just spent an hour on the phone, trying to choose an IB seat after booking the flight as an AA codeshare.

Simply put, this is not possible any more.

AA's record locator does not work on the IB website, and the EXP desk at AA can not give me the Iberia locator. The AA desk only has a five digit code, which is really a oneworld code, and not an AA or IB code.

I tried to play stupid with Iberia, as suggested above, and they could not even find my name on my flight, until I gave them the AA record locator. Then they found my reservation, but told me there was NOT an Iberia Reservation Code assigned to my ticket.

I'm going to try the 24 hour suggestion above for On Line Check In, but I really think I'm going to have to beg at the airport for my seat of choice two hours before the flight.

Makes me want to pass on code shares from now on...