[tbworld]

My reply is not going to help you, but it might give you a few ideas on how to go about doing a simpler version of what you are asking. Instead of trying to add this to the notification system -- which is tricky.

We created a new notification system instead for the groups and album system. When a new comment is added to a picture we simply display it in a special activity stream.
If the user has commented on the picture at all, in the pictures history, we highlight the comment in the stream for them.

We then send a simple notification to the main notification system with a link to the album page. This is not a descriptive link.

This is fairly straight forward to code without any real gotchas. Unfortunately, my code is owned by the company I work for. So I can only give you help and ideas here.