GTM iRule - answering DNS based on some criteria.

How about somthing like this:

when DNS_REQUEST {
    if { [IP::addr [IP::client_addr] equals 10.1.10.136/29] or [IP::addr [IP::client_addr] equals 10.10.10.64/27] or [IP::addr [IP::client_addr] equals 10.1.10.96/28] } {
        if {LB::status vs   up} {
            host 1.1.1.1
        }
        else {
            host 1.1.1.2
        }
    }
}

You may also try using a data group if you ever decide to expand or change the IPs your are checking for.