Actually in the past few months it has been easier to get 2 A seats out of LA for SYD-SCL than 2 D seats -- they've been selling out in J, but not in F. I don't know whether there is a difference in availability westbound vs. eastbound. Historically a phone call to LAN by your TA is often able to get A inventory released even when it is showing A0. This is easier this year as LA has historical data on past demand for this route so they are getting better about releasing it than when the route was new and more of an unknown.

As for ticketing, I've never done it ex-ARN but given that your trans-ocean sectors are 2 on BA and 1 on LA (or QF) you will have a very hard time getting anyone other than BA to issue this ticket. BA makes more than half of the revenue from your AONE4 ticket, not that they will act like they appreciate that.

As for flight availability to Japan in August, it is a non-issue, there are lots of available flights for you, and even the much preferred CX nonstop 744 service HKG-NRT is usually available in A every day.