Probably a question for gidgreen:
The history plugin catches the default 'c_post' event and defines if it is in_c_question (comment on question) or in_c_answer (comment on answer). However, it fails to identify a comment on comment.
I have described the issue here in detail: https://github.com/NoahY/q2a-history/issues/4
If somebody has a solution, I'd be glad to hear about! =)
PS: I receive a notification email of q2a that there is a comment on a comment, so there must be a way to find out... in qa-event-notify.php see c_commented_subject. It uses the array in the params called "thread", see foreach ($params['thread'] as $comment)
Another finding: qa-app-recalc.php → from line 316:
// For each comment thread, notify all previous comment authors of each comment in the thread (could get slow)
→ I somehow think that the best option would be just to add an in_c_question event for each user of the thread.