WordPress MediaWiki integration

Both MediaWiki and Wordpress support OpenID:

http://www.wordpress.org/extend/plugins/openid/

http://www.mediawiki.org/wiki/Extension:OpenID

Though, I think for automatic logins (after you log in to one, you automatically log in to the other) you would need to look into implementing checkid_immediate

http://www.openid.net/specs/openid-authentication-2_0.html#anchor28

The tutorial WordPress, bbPress & MediaWiki should get you on the right track to integrating MediaWiki into your WordPress install. It's certainly going to be a lot easier than hacking WordPress to have wiki features, especially with the sort of granular permissions you're describing.

Another solution is described in The CUNY Academic Commons Announces WPMu-MediaWiki Single Sign-on. It just creates something that uses the WordPress login as the master.

WPMW, a solution for integrating a MediaWiki within a WordPress installation, might help.