In this forum post, “rklopoto” describes how he runs pfSense WAN-less as only a DHCP server in an environment that already has a firewall. We didn’t think this was possible, it’s interesting what users run into!
This isn’t the best solution as essentially a DHCP server appliance, but it works. And he has good reason to do so – a single consistent interface to train administrators to use.
You do need two interfaces in the box, though you can leave the WAN unplugged. Then assign your LAN IP, and its default gateway as your WAN gateway. Just put in a fake/made up IP on a different subnet on the WAN page.
You can probably use a lot of different services in this fashion. Post a comment here, or on the forum or mailing list if you’ve done something like this.