custom_mp_buy_button_class.php

function custom_mp_buy_button_class( $post_id = NULL ) {
  global $id, $mp;
  $post_id = ( NULL === $post_id ) ? $id : $post_id;

  $meta = get_post_custom($post_id);
  //unserialize
  foreach ($meta as $key => $val) {
	  $meta[$key] = maybe_unserialize($val[0]);
	  if (!is_array($meta[$key]) && $key != "mp_is_sale" && $key != "mp_track_inventory" && $key != "mp_product_link" && $key != "mp_file")
	    $meta[$key] = array($meta[$key]);
	}

  //check stock
  $no_inventory = array();
  $all_out = false;
  if ($meta['mp_track_inventory']) {
    $cart = $mp->get_cart_contents();
    if (isset($cart[$post_id]) && is_array($cart[$post_id])) {
	    foreach ($cart[$post_id] as $variation => $data) {
	      if ($meta['mp_inventory'][$variation] <= $data['quantity'])
	        $no_inventory[] = $variation;
			}
			foreach ($meta['mp_inventory'] as $key => $stock) {
	      if (!in_array($key, $no_inventory) && $stock <= 0)
	        $no_inventory[] = $key;
			}
		}

		//find out of stock items that aren't in the cart
		foreach ($meta['mp_inventory'] as $key => $stock) {
      if (!in_array($key, $no_inventory) && $stock <= 0)
        $no_inventory[] = $key;
		}

		if (count($no_inventory) >= count($meta["mp_price"]))
		  $all_out = true;
  }

  if ($all_out) {
  		$all_out_class = 'mp_no_stock';
  }

  echo $all_out_class;
}