Stage one is to turn my modem into just an ADSL endpoint, removing any DHCP, NAT, and PPPoE termination from the device so that it has a single function.
Fortunately my nb604n ADSL modem has a nice easy-to-follow guide for taking it into bridge mode: http://support.netcommwireless.com/sm/videos/nb604n/nb604n-bridge-mode-setup-guide
Now onto greater things!