I'm in the same boat (albeit less years with status). I'll typically book BA for the redeye so I don't have to deal with the T5/T3 connection in LHR the following morning, and AA westbound for the LHR T3 lounge (and since I'll be protected by 261 either way). I have to have BA segments anyways since I'm frequently flying to European destinations that AA doesn't fly.
AA certainly prioritizes AA metal (over codeshare flights and other OW flights) for CK selection. However, I would think that the transatlantic BA flights could be the exception to the rule since AA/BA revenue share and coordinate schedules.