No, i don’t think you can just add the FDQN as a Balancing Rule. That’s a firewall table. What you have to do, i think, is write a bash or ruby or whatever script that adds the rule manually, after resolving the IP address.
You would want to put that script in Setup>Startup/Cron. And the script would have to:
* resolve the IP address with dig or nslookup
* extract it from the output
* manually add the rule with iptables.
And then schedule the job with the appropriate frequency. If it’s a site that realistically only changes IP once every few months, or so, than you might be OK with an hourly or daily job: in the worst case you would have to just wait until it refreshes, but only very rarely. Otherwise have it update every 5 mins or so.