I had a similar situation with CX on a 747 flight to CTS. I had a first class RTW ticket. Only biz is sold on HKG-CTS, and it was impossible to get a seat in the 1st class cabin in advance of the flight. However, on the day of the flight, CX had put everyone of us from biz into the F cabin.

Certainly if there had been more biz passengers, only some would have been moved forward. I was told that CX will make the decision, looking at their own elites and then looking at the oneworld elites. I don't think you can influence the decision other than being sure that your highest OneWorld status is associated with your PNR.