BuddyPress.org

This plugin kind of does it:

https://wordpress.org/extend/plugins/buddypress-group-forum-extras/other_notes/

as long as you follow the instructions under “Activity on Topic Posts”

Although it doesn’t actually show up as a forum “reply” like usual it looks like an activity update on the forum post here:

http://avatarthelastairbenderonline.com/groups/news-and-feedback/forum/topic/suggestion-thread/

You can see an example of what it looks like near the bottom of the thread.