How do other Macs in the network see my Freenas 9.3 backup drive attached to iMac via Ethernet?
Solution found:

On the iMac, we must first enable port forwarding by entering the following command in Terminal:

echo "rdr pass inet proto tcp from any to any port 12345 -&gt; 192.168.2.x port 548" | sudo pfctl -ef -


replace 192.168.2.x with whatever your FreeNAS is reporting as its IP address. You will also have to enter your account password when prompted. Now, in the Finder on your laptop, connect to your iMac's IP address (192.168.1.x) by typing command + k and entering afp://192.168.1.x:12345/. replace 192.168.1.x with whatever your iMac's IP address is, but keep :12345 as it's needed to connect to your FreeNAS. You may have to select the correct disk from a list. You should now be able to add it as a Time Machine disk on your laptop.