My experience at Epcot is that Morocco and China are wide-open for lunch in the slower seasons. Good opportunities at Germany as well. I have only eaten once at the Coral Reef, but it was fairly open also.
At Magic Kingdom, your best chance for a walk-in lunch is probably at Tony's, on Main Street.
I rarely (ever?) eat dinner in the parks, so I can't help with that. But I only go to WDW during off-season, so I have a lot of experience with that. Even in the off-season, I find that a lot of the resort restaurants are full during the prime dinner hours, but that one can get a seating even calling ahead by an hour or two to avoid the walk-up line.
The deluxe resorts almost all have a more expensive and less expensive restaurant - you may have a better chance at the more expensive place without a reservation. (California Grill is probably an exception to this.)
If you wanted to walk up for a Chef Mickeys dinner, arriving late (8:30 or later) may be your best chance. I enjoy looking down from the 11th floor, and there seem to be places open at that hour.
Hope this is helpful.