Apache's ErrorDocument directive lets you write a CGI script that will be invoked for all 404s.
IkiWiki could offer one as an optional wrapper; it would do much the same thing that the
existing recentchanges_link (or [[generic___39__do__61__goto__39___for_CGI]]) does when
encountering a nonexistent page.

I think it'd probably have to be a separate CGI script because the environment with which
404 handlers are invoked is somewhat odd, and because it needs to return a 404 status
(having said that, it might make sense for `recentchanges_link` to return 404 rather than
200 anyway if the page doesn't exist).

This would give IkiWiki the behaviour of many other wikis, where visiting a page that
does not yet exist prompts you to create it, without having to invoke the CGI for
successful requests.

Due to [a well-known MSIE misfeature](http://support.microsoft.com/default.aspx?scid=kb;en-us;Q294807),
error output needs to be at least 512 bytes long, so some padding might also be required.

I'm happy to write such a script if there is interest. --[[smcv]]