Thanks all for the help.
Yes, standing by doesn't seem a great idea - even if I get to LAX early my bags may not make it to SYD.
Good that you can see X5 (and I appreciate you finding the exact flight I'm talking about!). Unfortunately, I don't see any other flights on Aeroplan apart from one at 7am. I guess the X5 is for UA only. I guess it's still turn up at the airport and talk to whoever I can.
Still, good to know things haven't changed much in the years since I flew UA. Apart from not being called TED for all flights from LAS-LAX...(showing my age)