Note: This has only been tested with the default theme! Other themes will likely need minor CSS tweaks.
There are a few changes needed in Wakka.class.php, a new handler that needs to be created, and a couple of CSS modifications.
// handle body before header and footer: user may be logging in/out!
Create a new directory under the handlers directory called sidebar, and then in the new directory create sidebar.php and populate with the following contents. (I just noticed that the sidebar won't display if the user doesn't have read access...but then again, what would someone without read access use the sidebar for?) Also, you can change the SIDEBAR_CONTENT define to point to the Wikka page of your choice (currently, it's set to SidebarContent).
* Display a sidebar if the user has read access or is an admin.
* Cloned from show.php handler.
Unless I've forgotten something, that should be it!