You need to be aware that traffic routed to a load balanced gateway cannot use the system routing table, it all goes to the gateway. This means that if you have any other interfaces, OPT1 say, you won't be able access it from lan. If you need to do that you need a rule to allow it above the default any rule.
I'm sure there are many way to acheive external DNS blocking. I'm far from an expert myself, I await any other views. :)
Steve