Depends. If your TA or booking airline for the RTW can reserve the seat, then fine. Otherwise, you'd have to call CX res to ask them to release the desired seat for you.

I know that CX will release certain favourite blocked seats for MPC members. I do NOT know if they will do the same for other Oneworld elite members.

Lately I have found the desired emergency exit row seats on the 747 upper deck to be blocked out for general seat selection. A quick call to MPC released the desired seat.

If CX won't release the seats to you, then the desired cabin may just be blocked out and not released for seating at all. Otherwise you may check at the airport when checking in.