According to http://www.mediawiki.org/wiki/Manual:%24wgArticleRobotPolicies, one can set robot policies for individual pages with $wgArticleRobotPolicies.
However, it doesn't work for me, i.e., when I add:
$wgArticleRobotPolicies = array( 'Special:Categories' => 'index,follow' );
I still get noindex,nofollow on that page.
It is the same for other, non-special pages - setting anything with $wgArticleRobotPolicies for any page doesn't seem to have any effect.
Am I missing anything? I use 1.12.0.