It looks like the .json file is going to be the way to go, and I will update the guide once the dust has settled.
Much of it is still relevant, but the section below will effectively replace the Create iocage Jail and Configure Jail sections:
Create Jail (emby-server) from .json file
FreeNAS CLI
Code:
wget https://raw.githubusercontent.com/MediaBrowser/iocage-amd64/master/emby-server.json
iocage fetch -P --name emby-server.json ip4_addr="vtnet0|192.168.122.3/24"
Replace vtnet0 with your primary network interface (ifconfig will print them). Replace 192.168.122.3 and 24 with the IP and netmask you desire (should be different from that of your FreeNAS box). You can edit this afterwards in the jail configuration interface, but since the plugin needs internet access to install, the initial configuration should be valid.
After installation, I would restart the jail before creating the folders inside the jail and mounting your media datasets into the jail.
Code:
iocage restart emby-server
iocage console emby-server
Updating
FreeNAS CLI
Code:
iocage console emby-server
service emby-server stop
pkg add -f https://github.com/MediaBrowser/Emby.Releases/releases/download/3.6.0.2/emby-server-freebsd_3.6.0.2_amd64.txz
service emby-server start
Change the https:// location to the version you want to install, latest links can be found
here