How to set a subdirectory site for site-wide 404 page?

I have an MU network based on the zatz.com domain. When you go to zatz.com, it automatically takes you to the main info site, which is zatz.com/info. When you go to zatz.com/computingunplugged, you go to the Computing Unplugged site.

But when you go to a site that doesn't exist, or a page that doesn't exist (like zatz.com/palmpower), it takes you to the base zatz.com.

What I'd like to do is either (a) have a failed request always go to the page-not-found page of zatz.com/info or just to the main zatz.com/info, regardless of what the failed request requests.

Can I do this, and how?

Thanks!

--David