I've only flown AA codeshares on Aer Lingus, and all I can say is that we had no choice in our seats. I called the EI office immediately after ticketing and asked for 2 seats together and close to the front. We ended up in the 3rd to last row on the way to DUB and in the absolute last row on the way home. And both my wife and I are AA plat. I don't know if those were AA"s seats or not but they certainly weren't good ones.

It's worth noting that my this was the only time that my wife has ever said, "Whoa! I never noticed that MRTC thing until we got on that flight to Ireland and didn't have it..."