Module is incompatible with private files

I have the module working and rendering graphviz output to the graphviz dir. Unfortunately this module is incompatible with private files, so the images paths are not resolving. The module needs to implement hook_file_download($file).

I have a workaround (very ugly) along the following lines:

function xxx_file_download($file) {
  $file = file_create_path($file);
  if (strpos($file, 'graphviz')) {
    return array(
      'Content-Length: '. filesize($file),
    );
  }
}