The Phabricator API endpoint user.search should also output the value for "MediaWiki User". This would allow for tools and scripts linking the two. As a use case I'm working on a script that will pull new signatures from L37 (and others) and provide their MediaWiki usernames for vetting / adding to the ANPDP noticeboard. There is an endpoint that does the opposite (user.mediawikiquery). Perhaps there was a decision made to exclude this for some reason?
See also: https://secure.phabricator.com/T10512