My client has a Document Center. The whole document center make use of only one content type. One of the metadata on the content type is Classification level (Classified, Confidential, Top Secret). Everyone in the company can read all documents in the centre, except for those marked with metadata = 'Top Secret' documents. How can restrict permission on those type documents?
I'm sorry but it doesn't work this way and there are no reliable workarounds. You might be able to use event handlers or workflows to manage security when documents are created or updated but those things can fail, and item level security is a maintenance and performance headache.
When dealing with security your best bet is to use a separately secured site, or at least a secured folder.