Pay for upgrade
 

On the day of a flight (NRT-SYD), does QA offer a reasonable price for an upgrade to J? Doubt if being an AA PL has any impact on their decision. I was wondering if they would do it for a few hundred $ instead of leaving the seat empty.

No, but you are welcome to pay the fare difference to upgrade. This is likely to be more than just a few hundred $.

IIRC this can be done on BA? Vague memory of reading that in a thread sometime last year. Can anyone point me to it? (Mrs P and I will be flying BA longhaul in WHY in May)

In the past it was possible to pay for an on-board upgrade on BA (subject to space being available of course). According to the BA forum on FT, this arrangement has been discontinued. BA sometimes offer reasonably priced upgrades at the airport, but this seems to be only ex USA, and cannot be relied on.

I got a quote once, while I was waiting for the points upgrade to come through, out of curiosity - it was about AUD 10,000 from memory.

Dozens of airlines offer cheap paid upgrades in various forms; as mentioned the BA scheme has been axed (due to abuse). QF has never offered this, as a matter of policy (but there should be easy availability of QFF upgrades on that route). CX sometimes has paid upgrades but it would be about USD 1000 and not "a few hundred". Still a lot cheaper than the fare differential. But you would have to route NRT-HKG-SYD and the gate upgrade is only by sector. Don't believe JL ever offered it, either, for what it's worth.

Reminds me of the exchange in the original Beverly Hills Cop (when Eddie Murphy movies were actually funny, remember that?) :

Note that JQ Int (100% owned by QF) offers this.
Upgrade cost is typically the difference between the most expensive Y fare and the *C fare.
If this process was carried over to mainline QF would not be cheap