Here is the answer from last Saturday (lucky it was Saturday and not Monday.)

Flew DFW-SFO on AA, SFO-SYD on QF.

Received all my boarding passes at check in. The flight DFW-SFO was a Qantas codeshare.

Got to the security gate at the international terminal around 6pm. Got told that my AA issued boarding pass is no good, I can't get through security without a Qantas issued boading pass, and they wouldn't let me through.

They blamed Qantas for the rule, that Qantas will only allow their own paper/stock to be used.

Waiting an hour, twenty minutes at the Qantas Check-in desk for them to be finally opened (roughly 7:20pm) and then got my AA issued boarding passes swapped over to Qantas ones.

Went through security to the BA lounge, which was open. On the boarding pass it states to use BA lounge. And they pulled the plane up to the gate attached to the lounge, so we could board direct from the lounge.

I noticed some CX customers in the lounge, as well as a CX uniformed attendant, so this is the lounge that they use as well.

So in future, if I travelled via SFO again, would try and get into AA lounge until around 7:30/7:45 then move over to the international terminal to get the QF boarding pass.