It's very useful sometimes to have possibility to subscribe to particular discussion rather than overall forum, especially in big forums.
One example is this course: user, that ask a question about installation usualy want to receive e-mails about answer to his question, not all questions (or they can flood his e-mail box). General Developers Forum is another example.
Can this feature be added to standard Moodle forum?
Alternatively I can make changes by himself, but I have not access to the cvs and this require major change (new database table and so new version). Also I will need a period of time where no one will edit cron function and database schema.