First off, call the Executive Platinum desk and ask that they connect you to a US Airways Chairmans Preferred agent. You have that access with your EP status.
They may be able to contact CX to see if they would open up seats in F on another flight. Should CX not release F seats, do keep in mind that since the rest of your trip is in F, it will still be treated as a F ticket, so no miles back or compensation would be offered.
Also, you could possibly try calling CX directly. Can't hurt.