You ultimately may have to pay for a Meet & Greet on the "regular" tour. There has not been M&G's on the Christmas tour. The whole bus access thing is highly dependent on the venue. There are some venues where the buses are in the street, so there's not a whole lot they can do to stop you from standing near them. Having said that, chances are good that none of the girls will really stop and say hi. Mairead N used to be really good at taking a minute or two to greet people.
Almost...
There was a Meet and Greet at the Home for Christmas concert I went to in Saginaw (much to my surprise).
I know because I saw and talked to some people in the Meet and Greet group who had their fancy M+G stickers on (and presumably the PBS rep with the clipboard) and watched them go to the door to the backstage area a little while before the show.
I didn't think they had M+G's for HfC either until I saw it with my own eyes.