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

I have an MU network based on the domain. When you go to, it automatically takes you to the main info site, which is When you go to, 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, it takes you to the base

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

Can I do this, and how?