that is not really a very busy time of the year, as far as Disney goes...even Memorial weekend itself is do-able (and actually one of the lower-attendance holidays, ...there's a fair number of locals in the parks, yes, but mostly they're in line for rides, not in the table-service restaurants.) Majority of east-coast schools are not out for summer vacation until June, so it's just not that bad.
We have gone the week before Memorial Day and the week of Memorial Day including the holiday weekend many times on the dining plan...most restaurants were taking walk-ups (with the exception of the ever-popular character meals and a couple of the signature places.) So if you're not picky, you'll be able to find something...especially if you're willing to eat early or late in the meal time-slot (like 11:30 or 2:00 for lunch...or 4:30 or 5:00 for dinner.)