Thought I'd add something here as I've just struggled to get this working using information from the forum and the wiki.
As gnhb says above I have access to my modem without using any outbound NAT mappings or extra firewall rules.
I simply added an extra interface, assigned it to the NIC attached to my modem and set it to static IP in the same subnet as the modem.
The last step was to add an extra gateway on that interface with the modem IP address.
If someone would like to explain why the outbound NAT is needed here I'd be glad to know. For me it's working fine without it and adding it only caused problems.
Steve