I agree with the others that have stated you need ADR's for TS regardless of the time of day. The caveat would be when you are visiting. If it's a low crowd level time of year, you may be able to do walk-in's for lunch or dinner. However, it's not a guarantee. You might be able to get a walk-in table at Via Napoli, but you are going to have a significant wait for it. When we were there last March during Texas' spring break, there was an 1.5 hr. wait for a table at lunch. Needless to say, we did not wait.
If you are looking for a brunch option, I would check out the Grand Floridian Cafe for you MK day. Their breakfast and lunch menu is the same and is served all day until dinner service begins. You can get a burger or lobster Benedict. We like to eat breakfast in a resort before heading into the parks. We don't RD, so we tend to eat at "brunch" times. We really like Olivia's at Old Key West for breakfast.
In short, you cannot avoid scheduling ADR's for TS meals and maximize your DDP. If you are ok with using DDP credits on QS meals, then that's really the only way you can "spur of the moment" decide to eat, especially in the parks. I would not recommend that because of the amount of money you spend on the DDP versus the OOP cost of QS meals. Like melcort10 stated, there is a way to do what you're asking, but that's more of a possibility at resort restaurants.