Unless you are coupling things with a mileage run, you will probably want to fly AS on a separate ticket SEA-SFO/LAX. Otherwise you have to fly SEA-ORD/DFW-LAX, for example. If you can get to YVR easily, you might be able to time the QF YVR-SFO flight (which I don't think is daily) as a DONE4 alternative.

YVR could also open the potential for flying FCO-LHR-YVR on the way home on BA (if you care about AA accrual).

Steve