Interestingly, I just came across a commons link
included as if it were a
normal interwiki language link:
http://sk.wikipedia.org/wiki/Hrebienok
I guess this is against policy, but it's kind of cool.
When you think about it, it would actually make more sense for our links to
Wiktionary, Commons, Wikiquote etc to be displayed in the same way as links
to other languages.
I don't know if there's any policy or just nobody here thought of it.
Apparently the trick is that the template they use on the Slovakian
Wikipedia for project links[1] contains <div id="interProject"> and
their monobook [2] contains a javascript hack that puts such things in
the side-bar. Apparently this feature was copied from the Italian
Wikipedia.
[1]