I've never looked at paying for an upgrade before at check-in, but I'm going to try on an upcoming EWR-SIN flight, where the HKG-SIN segment has already cleared the GPU (which means it's gone) and the EWR-HKG segment is sold out in BF. I'm on a W fare, so I think it's unlikely to clear. I will do OLCI at exactly T-24, in case check-in time becomes the tie breaker (although I would probably lose to someone connecting through EWR who checked in earlier).
My questions: Does the offer come up at T-24 OLCI or only if you check in with a kiosk? If the flight is sold out at OLCI, but a seat opens later, will there still be a buy-up offer? How do you find out, other than repeating OLCI several times during the T-24 window?