This is why I read Flyertalk!
Last week, I booked a multicity itinerary through Europe on BA. I tried to book online, but with the BA website, it would never let me select an evening flight on one leg. So.... I booked over the phone. Agent asks me for my BA Executive Club number. I replied that I wanted to use an AAdvantage number instead. (I have status on AA, but not on BA, and wanted seat selection, etc.). The response was that that would be fine, but it is easier for him to book the ticket initially with the BA number, since all of the information- name, address, etc. are prefilled. So I give the BA number. At the end of the call, I make sure that he records my AA number. I wait a few hours and check the reservation, once it has been ticketed. No AA number, just BA. Wait another day. Still no go. Call the res line again, ask that it be changed. No problem, I was told. I go back, still no AA number. Call again the next day, same thing.
Then, I remember the trick that I read here about booking Iberia seats- the Finnair reservation site. I call up the reservation, and delete the BA number myself. Two hours later, voila! I can now select seats.
Whoever posted that little bit of info in one of the many threads that I have read, I am quite thankful right now! ^