If arriving internationally, you could probably do it prior to transiting to the domestic side. Once on the dometic side, probably not.

F seating is available to SOME C pax, and my experience that it is a FFP status and early check-in that win the day. I've flown twice with the family to HKT in C specifically to be seated in F seat, and not all of the F seats were allocated to C pax - which gleaned some confused looks from those sitting inthe regular C seat behind the F section - especially with my kids seated comfortably. This may have been by accident, but I doubt it.