Thanks for posting the link. We will be on this aircraft next year and it doesn't seem possible to choose seats. I tried online and also called LAN. Online I just get the spinning circle. The CSR said to call back next year - obviously trying to get rid of me.
We have seats booked for the connecting flight to GYE so no reason we shouldn't be able to book SYD-SCL.
I really wish QF flew on the day we need to travel so we didn't have to go through all this!