If, for example, you were trying to do this with USPS you could edit the shipping class:
/includes/classes/shipping.php

and add the code in RED:
Code:
  function calculate_boxes_weight_and_tare() {
    global $total_weight, $shipping_weight, $shipping_quoted, $shipping_num_boxes;

// bof: allow 1 zone only for Always Free Shipping (int)MODULE_SHIPPING_FREESHIPPER_ZONE
    global $db, $order, $cart;
        $valid_zone_id = (int)MODULE_SHIPPING_FREESHIPPER_ZONE;
        $check_flag = false;
        $check = $db->Execute("select zone_id from " . TABLE_ZONES_TO_GEO_ZONES . " where geo_zone_id = '" . $valid_zone_id . "' and zone_country_id = '" . $order->delivery['country']['id'] . "' order by zone_id");
        while (!$check->EOF) {
          if ($check->fields['zone_id'] < 1) {
            $check_flag = true;
            break;
          } elseif ($check->fields['zone_id'] == $order->delivery['zone_id']) {
            $check_flag = true;
            break;
          }
          $check->MoveNext();
        }

        if ($check_flag == false) {
          $total_weight += $_SESSION['cart']->free_shipping_weight;
        }
// bof: allow 1 zone only for Always Free Shipping
Then edit the USPS shipping module:
/includes/modules/shipping/usps.php

and add the code in RED:
Code:
  function update_status() {
    global $order, $db;
    if (IS_ADMIN_FLAG == TRUE) return;

    // disable only when entire cart is free shipping
    if (zen_get_shipping_enabled($this->code) == FALSE) $this->enabled = FALSE;

// bof: allow 1 zone only for Always Free Shipping (int)MODULE_SHIPPING_FREESHIPPER_ZONE
$this->enabled = TRUE;
// eof: allow 1 zone only for Always Free Shipping (int)MODULE_SHIPPING_FREESHIPPER_ZONE

    if ($this->enabled == true) {
Code:
// bof: allow 1 zone only for Always Free Shipping (int)MODULE_SHIPPING_FREESHIPPER_ZONE
//echo 'I see quantity: ' . ($_SESSION['cart']->in_cart_check('product_is_always_free_shipping', 1)) . '<br>';

        $valid_zone_id = (int)MODULE_SHIPPING_FREESHIPPER_ZONE;
        $check_flag = false;
        $check = $db->Execute("select zone_id from " . TABLE_ZONES_TO_GEO_ZONES . " where geo_zone_id = '" . $valid_zone_id . "' and zone_country_id = '" . $order->delivery['country']['id'] . "' order by zone_id");
        while (!$check->EOF) {
          if ($check->fields['zone_id'] < 1) {
            $check_flag = true;
            break;
          } elseif ($check->fields['zone_id'] == $order->delivery['zone_id']) {
            $check_flag = true;
            break;
          }
          $check->MoveNext();
        }

        if ($check_flag == true) {
          if ($_SESSION['cart']->count_contents() == $_SESSION['cart']->in_cart_check('product_is_always_free_shipping', 1)) {
            $this->enabled = false;
          } else {
            $this->enabled = true;
          }
        } else {
          $this->enabled = true;
        }
// eof: allow 1 zone only for Always Free Shipping (int)MODULE_SHIPPING_FREESHIPPER_ZONE

      global $template, $current_page_base;
      // CUSTOMIZED CONDITIONS GO HERE
then, on the Products for Always Free Shipping, mark them as such and add the weight that the shipping modules should use when the Product is not being sent to a Zone for Free Shipping ...

Now, in the shipping module for FREE SHIPPING! freeshipper, set your Zone that is allowed the Always Free Shipping on these Products ...