iRule redirect when 404

If I may add, I don't think the original description intends for an explicit redirect. As I understood it, Mimino simply wants to "fail over" to another server in the pool if the selected server returns a 404. If that's the case, something like this should work.

when CLIENT_ACCEPTED {
     set reqcount to the total number of servers in assigned pool
    set reqcount [active_members [LB::server pool]]
}
when HTTP_REQUEST {
     save the client request
    set req [HTTP::request]
}
when HTTP_RESPONSE {
    if { [HTTP::status] equals 404 } {
         if request count is greater than 0, decrement variable and retry request
        if { $reqcount > 0 } {
            incr reqcount -1
            HTTP::retry $req
        } else {
             all servers have been tried, so send a maintenance page
            HTTP::respond 200 content "Oops! You've asked for something we don't have."
        }
    }
}

If a 404 response is detected, another server will be chosen automatically. This iRule will then retry the original request for each member of the pool. If all servers return 404, then a static error page will be displayed.