imagecache folder and files outside the drupal installation path

hi!

i try to make imagecache save its generated files outside the drupal installation path.
i changed the system files path to "../" and i changed the imagecache file path with a theming function in template.php like

function blackborder_imagecache($namespace, $path, $alt = '', $title = '', $attributes = NULL) {
  $attributes = drupal_attributes($attributes);

  if (module_exists('transliteration')){
	$pathlength = strlen(file_directory_path());
	$splitted = substr(transliteration_get($path), $pathlength);	
	$imagecache_path = file_create_url(file_directory_path().'/imagecache/'. $namespace .$splitted);
	
  }else{
  	$imagecache_path = file_create_url(file_directory_path() .'/imagecache/'. $namespace .'/'. $path);
  }
  return '<img src="'. $imagecache_path .'" alt="'. check_plain($alt) .'" title="'. check_plain($title) .'" '. $attributes .' />';
}

but how could i make imagecache write its files to this path?
"../files/imagecache/namespace/images/"

can someone help me?