I think one way such a configuration can be done on pfSense is to set up your DHCP server to hand out the public IP addresses directly on the LAN (possibly bridging LAN with WAN, but I'm not completely sure). Under such a configuration, you would have it on manual outbound NAT and have the list of rules there cleared.
When your computers don't have public IP addresses, the router needs to change the source IP address because outside computers and routers don't know how to access 192.168.x.x addresses through your router. When your internal computers do have public IP addresses, outside computers and routers do know where to send packets to reach your connection, and thus no modification of the source IP address is required for the replies to arrive back at your router.
As far as I know, the router can say anything it wants for the source address when passing packets out from WAN to the Internet or out from LAN to your internal network. To get a reply, it basically only needs to be an IP address that the destination knows how to reply to. (assuming the destination is on a different side of the router) The router could probably even say that the source was just some random IP address on the Internet and the reply would go to that address, but the reply packets would most likely get dropped when they arrived because the computer or router at that address wouldn't be expecting the packets. (since neither side requested to open a connection with it)