PDF attachment missing (tmp directory location)

There is a major issue that may cause PDF files to not appear in the e-mail, regardless of proper configuration/installation as outlined on the project page / documentation.

More often than not, the temporary files directory may be located outside the docroot (e.g. it is common to use /tmp, while the website is located at /var/www/public_html). This means that the file can only be accessed with an absolute path. I assume this problem is not new, as it is documented in code:

function _commerce_billy_mail_get_attachments($order) {
  // [...]
  // Add a secong prefix slash as quickfix for this issue: https://drupal.org/node/760080
  // TODO - Solve cleanly, when the mimemail bug is fixed. TODO
  $pdf_path = '/' . $pdf_path;
  $attachments[] = array(
    'filepath' => $pdf_path, //you have to use relative paths.
  // [...]

However, for me the quick and dirty fix of prepending an additional / to the absolute filepath does not seem to suffice when the temporary files directory lies outside of the docroot.

temporary:// = /tmp (/tmp)    #FAIL
temporary:// = sites/default/files/tmp (/var/www/project/public_html/sites/default/files/tmp)    #PASS

Is this a specific issue for me or are you seeing the same?