How can I setup a "network billboard" on pfSense?

[AJungleDog]

Hi all,

I'm configuring a pfSense installation to use Dansguardian and squid in explicit-proxy mode.  I have it all working well.  Now I want to implement a "network billboard" as explained in here: http://contentfilter.futuragts.com/wiki/doku.php?id=network_billboard.  I'm pretty sure I could do this using a second computer running Apache but I'd rather just do it all on pfSense.  I already have the NAT rule to prevent circumventing Dansguardian and I believe I can easily modify this to send port 80 traffic to my web server.  My specific question is how can I configure the built-in web server in pfSense to display this page when the rule redirects traffic to it?  How do I duplicate the Apache statement: "ErrorDocument 404 /index.html" on pfSense?

Thanks for any help.

Brent