I'm not sure you can. I already opened a ticket because the MAC address changed every time the FreeNAS server is rebooted. This is likely true anytime the jail is stopped and started. If the MAC was consistent at least you could assign a static IP from your DHCP server.
https://support.freenas.org/ticket/1706
Currently the milestone is set for 9.1.0, which means we're 2+ versions away(My guess is xmas 2012 at the earliest but probably 2013) from being implemented by the FreeNAS developers unless someone in the community knows how to fix it and provides that information on the ticket.
If this is a big deal for you the only advice I can give is to comment on the ticket that this is a big deal and hope it is given a higher prority.