I've implemented a system for MediaWiki. 404 handler caching works by keeping copies of articles in a directory on the server in HTML format; if a page is missing from the directory, an error document (AKA 404 handler) is called by the server. If the 404 handler is MediaWiki, it can render the page to the client and cache that version to the directory, so that next time the Apache server will just serve the file directly. There's more on the technique here:

http://meta.wikimedia.org/wiki/404_handler_caching

Anyways, I did an implementation as a MediaWiki extension; the code is at http://wikitravel.org/~evan/Cache404.php.txt . There's no documentation to speak of; this is just an alpha version.

I've got a test wiki working with it on Wikitravel: http://wikitravel.org/test/ .

If you're at all interested, well, now you know.

~ESP