Thanks, Eric, for the post.
My opinion is that you should treat the PM system as private, but not secret. Private in the sense that it's not for the rest of the board to see. The conversations are between you, the distribution list and administration. Except that administration doesn't really care and doesn't read them unless there is a reason to care and read them.
I mean, really, the admins can read anything if they choose, whether the board software presents an easy way or not. It's all in a database accessible by them because it has to be.