As the only macro that includes the text "Email this post to" is "send_node_as_email_input", I guess that's the place to start.
I'm not a NAML expert but I guess it's just a matter of adding the code to insert a replyto header to the address of the current user.
Of course, the simple work around is to make posts by email, then it is easy to copy in any non users and their reply comes to you.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.