I looked at some doc for maildir, so I understand the format though your examples are very helpful. The question is how a user would link to a message and how you normally read such a message (what mode do you invoke). I would suspect you would want to link to the message when displaying it in some window, when on the file name in a dired buffer and potentially something like "link to most recent message from <user>". All of these require different bits of code.
Have you tried using Hyperbole's drag actions to create an explicit link to a file? Just type the label for the new button you want, select that label, depress the Action Mouse Key somewhere in the label, drag to the dired entry of the email message you want a link to and voila, you have an explicit button linked to that file. But to make it use some mode, you would need a custom action type that invoked the mode.
Bob