I just went through this yesterday with a new DIAL installation (http://dvswitch.org/files/AllStarLink/RAT_RC1.tar.gz).

As you have observed, netsetup (for Static setup mode) sets the DNS nameservers in /etc/resolv.conf but updating that file directly is not supported for the OS. On reboot, the resolvconf process overwrites /etc/resolv.conf again and removes those entries.

And as you also found, entering dns-nameservers in /etc/network/interfaces doesn't work with resovconf.

I've decided to use netsetup to set networking to DHCP and I set a static MAC entry for the IP I want assigned in my router. This works as expected.