During our Christmas week '08 trip, I don't remember any character meals where they were dressed specially. We ate at LTT (not character any longer) and 1900 Park Fare that trip that I can remember.
This isn't going to help much...but I ate at LLT during the MNSSHP and the characters in the restaurant were dressed for halloween. I haven't been to Disney during MVMCP so I have no idea if it is the same. Oh...and I went to the Halloween party about two years ago so that may have changed. I know there isn't character meals at that restaurant anymore.
We did a few character meals during our December trip last year, no costumes. Now some of the characters in the parks are dressed up, all the characters(mickey etc) were dressed up in Animal Kingdom.
We ate at Akershus last year, mid-December and none of the princesses were dressed for the holidays. It's a really cool idea! I wish they would, you should suggest it.
LTT when they had characters did dress up for Halloween .I think they had stuff for Chrsitmas too.They no longer have characters so it doesn't matter..CP didn't and CRT didn't