I've actually done this -- flown QF on AA codeshare -- and there is no way to preassign seats prior to airport check-in. Being AA EXP and QF WP was no help. Years ago AA got a contiguous block of seats on QF but this is no longer the case, and AA codeshare seats are sprinkled amongst the QF seats on the plane. As you suspect, the better seats go to the QF pax while the AA codeshare pax get the bottom of the barrel, except for Emeralds who get the CL/WP1 reserved but unused seats made available on day of flight. Bottom line is that if seat allocation is important for you, buy the QF code. If you don't care, buy the AA code. Was AA cheaper? In the past pre-bankruptcy days the AA codeshare on QF was typically 10%-20% more expensive, tapping the US gov't contractor market who were forced to buy American even if higher cost; AA in-bankruptcy seems to have lowered some of these fares to generate more revenue.