Still not working. Is there a particular place it needs to be defined? I tried it in /includes/extra_datafiles/middlebox_filenames.php,
/includes/languages/english/extra_definitions/YOUR_TEMPLATE/middlebox_defines.php
and
/includes/languages/english/extra_definitions/middlebox_definitions.php

Does it need to be defined before the SQL patch is run?

Ah yes, of course, define it in admin :)