You can make a priority seating (ps) up to 90 days before you want to eat there by calling 1-407-WDW-DINE.
We plan on eating lunch at the Crystal Palace also. There isn't a lunch with Cinderella, but you can eat lunch in the castle. I've heard that Cinderella is often in the lobby to greet guests and take pictures, even if she isn't in the dining room. There is a dinner at the 1900 Park Fare restaurant at the Grand Floridian resort with Cinderella, Prince Charming, 2 of the mice, and the Fairy Godmother (she's in the lobby for pictures only) if that helps any.
Good luck!
Steph