If Qantas do not allow you to check your bags in more than 4hrs ahead, which I would be suprised if they wouldn't (you should be able to check them through from SFO) then you could always put them in the short term storage lockers and then take off for the day, can think of much better ways to spend a day in LA than sitting around the airport.
Best bet I think, is to just check your bags all the way through and then take off (I've done it before) just as long as both your flights are with eiher qantas or oneworld carriers.