It's also not difficult to add links to sister projects in the
sidebar, as seen at en.wikibooks since February 2011 [1]. And I know
some other projects do similar things as well, some with JavaScript.
The only difficulty encountered at en.wikibooks is not knowing how to
push the print/export box above the sister project links.
[1]
http://en.wikibooks.org/wiki/MediaWiki:Sidebar
-- Adrignola