This is where I would skip the award ticket for the domestic leg and just buy a cheap one-way ticket from LAX to whereever you are going.
SYD-LAX is long enough of a flight. I wouldn't want to deal with change of airport and the 3+ hours in traffic to go from LAX to ONT. The only feasible way to get there is one-way rental car or Uber, both methods will cost about $200 plus meals and hotel for your stay in town. So you are probably looking at $400-500 out of pocket cost for this "free" award ticket. How much is a one-way ticket from LAX to your final destination?