This feature has now been released.
Only site owners have the ability to enable this feature, which is
turned off by default for new and existing sites. To turn on
page-level permissions, go to More Actions > Sharing and
Permissions.

From there, click Enable page-level permissions. Then, in the
dialog box, click Turn on page-level permissions.

Once page-level permissions is enabled, you’ll have three options to
choose from:
- allow a page to inherit all of your site-level permissions
- elect to include future site-level changes to a page
- prevent a page from inheriting any future changes made at the site-level

Using page-level permissions should give you greater control over who
can edit and access your Google site. To learn more about setting
page-level permissions, take a look at our getting started guide.
Let us know what you think in our support forums.