How would I change a Special Page (User List) to a Restricted Special Page?
I think you just add something like:
# Permission check if( !$wgUser->isAllowed( 'block' ) ) { $wgOut->permissionRequired( 'block' ); return; }
(copied from SpecialBlockip.php)
to the constructor (which is right at the bottom of SpecialListUsers.php). There must be something else required to stop it appearing on Special:Specialpages, but that should be enough to stop users without the appropriate permission (replace block with whatever you like) seeing the list.