You should get an email right before check-in that's exactly like when you have an un-cleared upgrade instrument ("we could not confirm your upgrade on...") and you should be added to the upgrade standby list on check-in. Agree with Kacee that if that happens, the system is working right (whatever your priority) and you should report back how high on the list you are.
I am in the camp which believes this priority should place you after all instrument upgrades, which on an int'l route means you should be at the bottom of the list except for NRSA.