I've improved my wiki-links hack by implementing the other half of standard wiki-link functionality: if there isn't already a post/page with the given name, the name will be displayed as normal text (not a link), but there will be a "[?]" marker after it. Clicking the marker takes you to the new-post form. The marker will only be displayed if the user has permission to post, so casual viewers won't see any clutter; they won't even know the potential wiki-link.
I also fixed a bug in the main regular expression, and added the necessary boilerplate to make this work as a plug-in in WordPress 1.2.