Yes, the Plaza Restaurant is open for dinner with the same menu as lunch. Here is my review found in my soon-to-be-released "The Luxury Guide to Walt Disney World".
Plaza Restaurant $
American Cuisine. Lunch and dinner.
A pleasant ambience permeates this informal restaurant located at the castle end of Main Street. Servers in Victorian dress mill about the Art Nouveau dining room serving simple but tasty sandwiches, hamburgers, and salads. From the veranda seats are impressive views of Cinderellas Castle and the moat surrounding it. If not for a meal, stop in for one of their famous ice cream specialties.
Sample Menu Items
Lunch & Dinner Entrees: Rueben Sandwich, Plaza Club, Chicken and Strawberry Salad (garden greens, grilled chicken, fresh strawberries, gorgonzola cheese, and red onions tossed in a white zinfandel vinaigrette), Create-Your-Own Hamburger, Fresh Vegetable Sandwich