This is exactly the eventuality that EZ404 (http://www.zope.org/Members/abgillette/EZ404) was created to handle.

Sure, but now we are talking Zope. And with Zope, directory/ and directory/index_html is usually the same thing (and should IMO always be the same thing).

...

Additionally, I don't have much influence on the way other people create external links to our webpages :-(

Exactly. Thats why redirections are A Good Thing. But they should be done right. There is no reason to configure the web so that the new site has exactly the same URL's as the old. The old rather should redirect the old URLs to the correct place, instead of redirecting the new URL's to the old place.

See for example http://www.regebro.nu/lennart/NetWare.html, which redirects you to http://lennart.regebro.nu/netware/ No problemo. Very nice, and very easy to do with Zope. That is the best way to handle old link: Make sure they work, by redirecting them to the new links. Transparent for the user, easy for the webmanagers and gives you the correct, new URL.