It always amazes me when I see that people have gotten in as a walk up! Every time I've been to BOG I've seen people trying to get in without an ADR and every time they are turned away. I'm sure it does happen, but I've never personally seen it.
Ohana is another one that I always see people trying to walk up too. Every.single.time I've been there. This past May a lady was full-on arguing with the check-in CM for 10 minutes while I was right there waiting to check in. She was telling the CM it wasn't fair she couldn't get an ADR (she apparently tried). Her family just walked away after about 5 minutes, but she kept going on.
OP, I think you're better off either using the TP reservation finder or trying the night before when people dump them before they're charged.