GUI? Not a chance before FreeNAS 10. The 9.3 GUI is frozen for all but essential and/or small changes. The FreeNAS 10 GUI will be much more flexible, so a feature request might get the machine rolling.
It's not trivial, but doable. If CLI is ok, you can hack something up yourself with the output of ipmitool sensor -listall (or however it went, check the IPMI sensor threshold guide part 1).