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.
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.
Mark