Carbonchaser - No, they could not as they were massively overbooked for the next 3 or 4 days. They could not get us a seat on any LAX or SFO flights. They also could not get us on a future SFO - HNL - SYD flight.
In hindsight we would not have taken up the option.
When we volunteered we were going SFO - HNL - SYD - MEL, getting us into MEL at 7pm instead of 11am same day as SFO - SYD - MEL flight.
After removing us from flight it was "Oh, that flight is now full, we'll get you there another way". We did not have the ability to retract our acceptance of the offer. United basically changed the terms of their offer once we were removed from the list.
The flight from Narita to Melbourne was absurd. 10 hours on a QF 767 with abysmal crew and service. We were 5 mins from landing at MEL when fog forced us to turnback to SYD. We were parked at cargo area in SYD for 2.5 hours, not allowed to use mobile/cell phones, no food, no drinks except tap water from plane. Eventually we flew to MEL and arrived some 6 hours after we were scheduled. An absolute disaster.
Finally, three bags were damaged and because 2 were tagged heavy (some 2 kilos over) Qantas refused to cover the damage because the bags were overweight. United on the other hand would have covered the bags.
I'll write again and if I get no luck I'll call and speak to a supervisor.
Thanks again.