Thanks for the comments above. It never occurred to me that the AONE4 fare rule could be applied to avoid compensation for a downgrade after ticketing. I have now received confirmation that such an interpretation of the rules is false.

According to QF Executive Relations in Australia, compensation is payable as this was an involuntary downgrade and since I was originally ticketed in F. She confirmed that the inaptly titled "Customer Relations Manager" in L.A. had erred in applying a fare rule in this case. Compensation of $1100 has been paid, apparently based on the fare difference between F and C on the SYD-JNB route.

I also received an apology for the way that the entire episode had been handled, and a voucher for $400.