There's always a chance, but no guarantee that you'll be accommodated. People have come back and reported on the disboards that they were able to walk up. The night we were there last month walk ups were being told to check back periodically during the evening. During the time we waited, I did not see any walk ups being checked in and given pagers. BOG was full the entire time we were dining.
Lunch is counter service and doesn't take reservations so if you can't get in for dinner, then lunch is something to consider. The menu is different, but very good and you'll be able to walk around to see all the rooms.