DNS response validation?

I'm thinking an iRule is the right way to go here. I'll do something like throw both sets of authoritative servers for that reverse zone in a pool, then when DNS_Response if DNS::ptype =="NXDOMAIN" try next member of pool. Then iterate each member of the pool and if none of them return an answer then return nxdomain to the client. Is that something that is possible? Note that I'm using SNAT auto mapping, since the clients aren't routed through the F5. I'm new to iRules, should the rule begin with DNS_Request then if DNS_Reponse and nested DNS::ptype? Not sure if the rules watch the whole session or not.