It is currently not possible to specify a plugin file with a separate jail name.

 -P, --plugin-file               This is a plugin file outside the INDEX, but exists in
                                  that location.
                                  Developer option, most will prefer to use
                                  --plugins.
  -n, --name TEXT                 Supply a plugin name for --plugins to fetch or use a
                                  autocompleted filename for --plugin-file.
                                  Also accepts
                                  full path for --plugin-file.

Current behavior/way to do it.

iocage fetch -P dhcp=on vnet=on bpf=yes allow_raw_sockets=1 -n emby-server.json --branch 'master'
iocage rename emby-server emby-server-pleasedontdestroy

Suggested modified call:

iocage fetch --plugin-file=emby-server.json dhcp=on vnet=on bpf=yes allow_raw_sockets=1 --name emby-server-pleasedontdestroy --branch 'master'