I just got a CX J award MELxHKGxSFO with an AS segment SFO-SEA. The availability was showing on QF but not on BA. I originally just asked for the CX flight, but when the agent had found that, I asked about adding SFO-FCA. He checked and said that wasn't a valid routing, and priced it out separately, but then I thought about just going to SEA, & that was available for the same points, so I grabbed it. I'm waitlisted for an upgrade on the AS leg, but as far as I can there isn't a big advantage over Y (maybe lounge access at SFO and food on the plane).