Ok, I am writing the last bits of this today.
I decided based on this thread to do the following:
1. A webuser for each subscriber (optionally blocked as default).
2. A webuser group for each newsletter so that the items mentioned in this post (
http://modxcms.com/forums/index.php/topic,1356.msg8999.html#msg8999) are possible.
3. A table containing info for each newsletter including name, description, tpl (template html body code), active, webgroup (id of the webuser group that the newsletter is tied to), createdon, editedon, lastsent