This is a nice idea and....Until admin makes this a feature, in the meantime, we can redirect people to certain pages with .htaccess---

The user pages are "virtual" and the format is always http://yourdomain.com/joeblow

So, in .htaccess you could put something like this:

Redirect 301 /joeblow http://yourdomain.com/joeblowerrorpage.php

More on htaccess redirect here: http://www.webweaver.nu/html-tips/web-redirection.shtml