This is just based on my instinct from previous experience but I think that if you don't get the upgrade during OLCI, then all seats in the F bucket are either sold and/or checked-in. I suspect that if someone who is holding a seat in the F bucket changes or cancels their flight thus opening up that seat, there may not be enough time for the EUA to run and upgrade you. Also, it may be possible that the "lock in" means that once you've OLCI'd and didn't get upgraded, the EUA would bypass you in the event an F seat did open up. Again, I don't know for sure on that point.
Maybe someone can confirm that if it is within the 24 hour window and the EUA has run and upgraded everyone it can, if there are no seats left in the F bucket, then anything that does open up will only be done at the gate and not via a last minute EUA run.