Additionally to the above, if you have a twitter account you could tweet to @DisneylandToday and ask if Cinderella will be in the parks that day. I'm sure they get a billion tweets so you may not be answered, but they just answered someone asking about Peter Pan so they do respond from time to time. I wouldn't bank on it, but it couldn't hurt either.
Probably would only make sense the morning of, though, not a day in advance or anything.
Edit: and they just responded to another tweet asking about Aurora's availability...