How can I use the Comment field as a way to privately message the node author?
I see some potential pitfalls with this approach:


It probably won't be quite as simple as you describe because I guess you'd also like commenters to be able to see their comments, and replies to their comments, while hiding other people's comments. (Or am I wrong?)
You mention you'd like to allow anon users to comment, so you need some way to differentiate one anon user from another (if they're able to see their own comments, but not others).
It won't play nicely with page caching (at least without a bit of effort).
It might make life a bit more difficult if you want to have normal comments on a content type at some point. (Eg. should the comment moderation tools be showing comments which are actually private messages?)


If you do want to go down this route however you can use hook_node_view_alter().