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 'do=goto' 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 turns out to be untrue, as long as the wrapper lets a couple of extra environment variables through. --smcv
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, error output needs to be at least 512 bytes long, so some padding might also be required.