The BA trick just worked for me. I just used AA miles for an itinerary with two IB segments and two AA segments. I went to manage my booking at BA.com, entered my AA record locator and the last name on the reservation, and it pulled it up. It showed a different record locator, which I was able to use on IB.com to call up the reservation and select seats. So, at least for me, BA can see the reservation using the AA record locator even if there are no BA segments, and it displays a record locator that works with IB.