This feature has been added to PFSense in CVS recently however it will not show up until version 1.3 since PFSense is feature locked on 1.2.
I have four young boys 3-8 years old and I'm am using Opendns filtering for parental control with PFSense.
Anway if someone wants to add it to m0n0wall it could be pretty easy to add it since PFSense is based off of m0n0wall.
Here are the pfsense check-ins for dns-o-matic if that helps. The check-ins can provide the basis for the code needed.
http://cvstrac.pfsense.com/search?s=dns-o-matic&t=1&c=1I wanted the feature now for my pfsense 1.2RC3 system and so I manually applied the dns-o-matic checkins to my firewall. I will look into adding it to m0n0wall my main problem was access to the files the way I've gotten into pfsense config was using ftp over ssh (sftp) not sure how to do that in m0n0wall.
Mark