I think you might be able to do what you want using the new FlaggedRevs
extension, rather than having an extra tab you would be able to have an
'reviewed / approved' version of the page ('as is') that is the default
page that is seen, and then a 'new version' of the page that would
contain suggested revisions etc ('to be').
You can set permission based on logged in / unlogged in, membership of
editors / approvers / readers groups etc.
One advantage is that this is an extension and doesn't require 'hacking'
of any of the standard code
http://www.mediawiki.org/wiki/Extension:FlaggedRevs
Ta
John