see
http://wiki.splitbrain.org/wiki:discussion:sidebar_example
and implementation
http://wiki.jalakai.co.uk/wiki
sidebar attempts to display a "navigation" page (name configurable) for the current namespace or if not found, its closest ancestor. If there are no navigation pages it will display the index at the namespace level of the current page. The navigation page is a normal DokuWiki page.
note, the above implementation can generate bad html if the "navigation" page has a TOC. Strictly a facility needs to be added to the parser to be able to turn off the TOC while it generates the HTML for the navigation sidebar.