I've written a small patch the does the following things for every language pack in Moodle:
1. Copy any strings defined in auth_*.php that are not in auth.php into auth.php
2. Create auth_*.php files if they don't already exist based on the files in en_utf8
3. Copy module specific strings from auth.php to auth_module.php files based on the strings as they exist in en_utf8
This ensures that all language packs have a language file for all auth plugins, and that all auth strings are duplicated in auth.php to ensure backwards compatibility.
If someone could please have a look at the patch just to make sure everything is OK. I've also attached the script I used encase anyone in the future needs to do something similar.