Separate Site Strategy
Given your clarifications, have you considered creating a separate site on which to host this protected content? In the site definition you can specify the security domain, and therefore limit it to Sitecore users/authors.
extranet\anonymous is different from
sitecore\anonymous Sitecore's user names are actually stored in ASP.NET Membership Services as fully-qualified names, including their domain. Sitecore simply looks at the supplied user name, and if it doesn't include the domain portion, prefixes the user name with the current Site's domain. (
extranet by default on public sites)
Your login form should support allowing Sitecore users to enter their user name as "sitecore\myUserName" I have several clients that use this strategy currently, as it saves them from having discrete accounts to edit and read protected content.
In order for this to work correctly, your
sitecore\Everyone Role (or other more limited and appropriate role) needs the Read right to the Items you're protecting. If your content authors can currently edit the Items, you're all set, but if your reading Sitecore users are different from your editing Sitecore users, you'll have to make sure the permissions are set appropriately.