iRule checking Pool is down

Now I understand what you were looking for, so why don't you simple do this way:

when HTTP_REQUEST {
    if { [catch { eval [LB::select] }] } {
        HTTP::respond 200 content {
            ************************************************************************************************
            E-R-R-O-R: Sorry - no Node with status ACTIVE or DISABLED found!!! 
            ************************************************************************************************
        } Connection close
    }
}

Thus, when no members active in pool, the new connections will get sorry page immediately, and for the member disabled the current connections keep working till it end.