I have a network connection with a static IP and a headless Linux box with dual network cards between my LAN and the rest of the world. My laptop is using the OSX version of Limewire.

As I recall, the Windows version of the various Gnutella variants had an option for forcing your IP address that allows you to specify a hard-coded IP and port. The OSX version does not seem to have a place for the IP. For instance, I can select LimeWire -> Preferences -> Advanced -> Firewalls. In that dialog, I get simply a checkbox for "Force IP" and a text field for a port number, but there does not seem to be anywhere to enter the firewall's external IP address.

Is this functionality missing? Is there a different place to plug in my IP address? Is it "automagically" detected from packets and used by remote hosts?

You're right that it's picked up from somewhere, but the method has varied, I recall. You can see it listed if you look at one of the lines in the Sample Bug Report.