Similar to how $wgNoFollowLinks, $wgNoFollowNsExceptions, and $wgNoFollowDomainException can be used to set rel="nofollow" on certain external links, there should be a complimentary set of config options to set rel="noreferrer".
A discussion is ongoing on enwiki about adding noreferrer to external links for privacy reasons (http://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)#noreferrer_for_Wikipedia). This will give them the software option of doing so if they decide.
Patch soon to come.
Version: unspecified
Severity: enhancement
URL: http://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)#noreferrer_for_Wikipedia