Short haul: F to PEK is always wide open for redemption. However, it is more financially sensible to redeem it outright than to upgrade. HND occasionally has seats open. You might be able to FLUB on certain routes but that is a long shot.

Long Haul: LHR has 1-2 daily flights with F. That being said, F to and from LHR is almost always full months in advance. Instead of chancing it with the waitlist black box, finding award seats might be you best bet. Last time I checked, CX released a bunch of LHR F seats for Q4. The daily CDG has F but that might change in the future. I can’t comment on NA flights as I rarely fly that direction.