It quietly happened in the mediawiki release last week {git #f0d86f92 - Add additional interwiki links as requested in various bugs (bug 16962, bug 21915)} [1] that the mul: interwiki has appeared for the wikisources to be able to point to wikisource.org. So we can now do [[:mul:XXXXXXxxxxxx]] and it appears in interwiki links as "More languages". We will have a number of places, especially in the project namespace where we need to do some additions.
Now to see how we can get mul: working for Wikidata.
Regards, Billinghurst
[1] https://www.mediawiki.org/wiki/MediaWiki_1.24/wmf22#WikimediaMaintenance