Allowing a single page to be public on a private blog


On most of our sites we have multisite privacy set so that only registered users of the blog can access the site.
This works very well. (thanks!)

However, we have a situation on one blog that the blog owner wants a single page to be visible to anyone (open access), but the rest of the blog to be private.

Do you know of anyway to enable this? I'm guessing there isn't an interface for this!
Thanks in advance,