If you want to reply from your own email, you click the "reply" of your email and delete the stuff in the digest that you don't reply to, and change the subjectline to that of the thread you are replying to.
and apparently, that does work, AND gets threaded properly.
I'm not sure whether the line "Do not reply to this email. To reply to one of the messages in the digest, click on the link at the bottom of the message." should be changed