Keep SFO-SYD in mind as a fall back option too. Sometimes LAX flights fill up more quickly for some reason.
Oddly, if you're looking at economy loads, the QF system of upgrading frequent flyers actually works in your favour. At -24hrs if there are any business or premium seats available, they will usually be taken by people who were previously occupying an economy seat, thus freeing the seat up for you.
If you haven't already, check out
http://stafftraveler.com - very useful for obtaining loads for interline carriers.