When I checked in with QF today in SFO, I was told that I would have access to the Sakura lounge. I noticed that BA Terrace was immediately adjacent to the departure gate, so gave it a try. A BA flight had just left so the staff were cleaning up, restocking, etc. Nobody at the desk. Finally, a very pleasant chap appeared and I asked if they were open and could I use Terrace as AA PLT. He looked at my boarding pass (I am flying in Business) and said, "Sure, why not. The place is almost empty." We had a nice chat comparing Terrace with Sekura. Earlier in the day, I spoke with AA, who told me that I could only use BA Terrace as EXP or when flying in first.

Never hurts to ask!

And I scored row 16 upper deck on QF 74.