Somewhere between slim and none
The reason you can't book an ADR is there is no room.
If there is no room for an ADR then there is no room for a walk up.
BUT, folks cancel all the time. And even with a fee, there are no shows. So it never hurts to ask.
Also, it may be worth trying for 1 and close together times. Since it's self seating for lunch, party size doesn't matter. Usually, Disney is harder to find ADR for 1 but it may not be the case for BOG