Last time I ate there, I saw people walking up about asking about wait times, but keep in mind this was right around Sept, so not too many people at the resorts yet.
For most places, you can inquire. Always walk up and inquire. You'd be surprised how many restaurants you see not available and you can just put your name in. Granted, places like Ohana and a few others you may find hard to get by walking up. Also, even if you see the Walk Up Availability in the app only showing a few restaurants, STILL walk up and ask to see how they're booked for the day. Disney does not want to turn away a guest who wants to dine and spend money, so if there is room they will let you know.
I love Homecomin'. You can only find it when you book 60 days out early in the morning online, butttt Homecomin's walkup is always 40min or less from my experience. I originally thought this was a not walk-up available type restaurant and its reservation only, and since you can't find them online, I just assumed. But sure enough I saw others walk up asking about the wait. I never book it via Disney anymore, I just walk up and assume it could be 60 min wait or less at Homecomin'.
Be Our Guest I can see having zero availability, but that's a given, and even then I see people walking up and asking just to see.