Each bylined author of a blog post should receive an email notification when a new (approved) comment to their post is published.

Note that the standard WordPress mechanism for that won't work here, as we are using the "Co-Authors Plus" plugin to assign authors to posts with guest accounts:
http://vip.wordpress.com/documentation/add-guest-bylines-to-your-content-with-co-authors-plus/
(even for those that have a separate "real" account as blog admins, e.g. I access WordPress and edit posts using the "wikimediablog" account, but posts appearing under my name as byline are attached to the separate "Tilman Bayer" guest account). So we will need to ask Automattic whether the email contact info for guest authors can be used for that.