Sorry to dig up this old thread.

I am in a similar situation to the OP and wonder if there is any way I could near-guarantee its use on a long haul (HKG-MXP Y-J, Oct/Nov), whether by going to the airport hours early, OLCI or any other method in order to secure priority over paid upgrades, reward upgrades, op-ups, etc. A few questions :-

(a) Is there any definitive priority queue for this?
(b) Would the underlying booking class matter? (Can I exercise the certificate on a V or even lower booking? Would there be any situation in which use of the certificate is denied but mileage upgrade permitted - which would require K or above?)
(c) Would timing of arrival at airport matter? (For example, presumably would have to get to the airport before the op-ups are processed, but how about versus mileages upgrades who arrive at the airport earlier? Would these be finalised before check-in closes?)

I nearly used it on a short-haul this weekend (in which I knew that J was only 70% full) but was still told by the check-in desk in HKG that "the flight is full but I will make a request and if it goes through, you can exchange boarding passes and certificate at the gate". I didn't use it in the end but J (and Y) took off with plenty of empty seats. Just curious if anyone knows whether this is the SOP for this certificate.

Would be great to hear from anyone with relevant experience, thanks!