Como prometi, refiz o script para várias redes, retornando DIRECT só para localhost e endereços dentro da mesma rede do cliente.
function FindProxyForURL(url, host){
var host_ip;
host_ip= dnsResolve(host);
if (isInNet(host_ip, "127.0.0.1", "255.255.255.255"))
return "DIRECT";
if (isInNet(myIpAddress(), "192.168.7.0", "255.255.255.0"))
if (isInNet(host_ip, "192.168.7.0", "255.255.255.0"))
return "DIRECT";
if (isInNet(myIpAddress(), "192.168.12.0", "255.255.255.0"))
if (isInNet(host_ip, "192.168.12.0", "255.255.255.0"))
return "DIRECT";
if (isInNet(myIpAddress(), "192.168.13.0", "255.255.255.0"))
if (isInNet(host_ip, "192.168.13.0", "255.255.255.0"))
return "DIRECT";
if (isInNet(myIpAddress(), "192.168.14.0", "255.255.255.0"))
if (isInNet(host_ip, "192.168.14.0", "255.255.255.0"))
return "DIRECT";
if (isInNet(myIpAddress(), "192.168.15.0", "255.255.255.0"))
if (isInNet(host_ip, "192.168.15.0", "255.255.255.0"))
return "DIRECT";
if (isInNet(myIpAddress(), "192.168.16.0", "255.255.255.0"))
if (isInNet(host_ip, "192.168.16.0", "255.255.255.0"))
return "DIRECT";
if (isInNet(myIpAddress(), "172.16.0.0", "255.255.0.0"))
if (isInNet(host_ip, "172.16.0.0", "255.255.0.0"))
return "DIRECT";
return "PROXY 192.168.7.1:3128";
}