Well I am thinking about reward flights, but that's stage 2. You certainly can't book a reward flight in F when there isn't any paid availability.

I know CX is flaky in respect of last minute equipment swaps, and that's a calculated risk I would be taking. If F availability ever does comes up on a 777, I would be prepared to book, with the logic that as a 4 cabin plane, it's less likely to be swapped out than a 3 cabin plane with no PE. I would book estimating there was probably a 50% chance of actually flying in an F seat. I wouldn't book F on a 3 class A330, as with few bums on seats in F (for the reasons you outline above) and no PE to worry about, that's a much lower hassle equipment-swap target.

The annoyance level when downgraded on a redemption flight, when you will get back an incontestable quantity of avios, is much lower than if downgraded on a paid-for flight, when Cathay generally calculates the refund due as a derisory amount.