is there any support for this (www.dtdns.com)?

It would (could (or something likethat)) work with just single line of 
http:// and proper ?id=xx&pw=xx&ip=xx, so it could be easily implemented?

Specs for updating dtdns can be found from: http://www.dtdns.com/specs.txt

And yes, it can be done easily outside of m0n0wall, but I think that it 
could be also done within m0n0wall..

- Harri