As requested, it is now possible to leave Private Comments.
When you leave a comment you'll see a new tickbox, which if you tick it, means that ONLY the author of the diary can read the comment.
In order to read the comment, the diary author will need to login, using the login button at the top of the page, as if they were going to add an entry... In theory, the rest should be simple :-)
Give me a shout if you get stuck or have any questions,
Steve.