I think I actually have it working the way I want, but can you confirm a few things for me:
The mail triggering looks at the individual meta s2_cat* having a value of something (in most cases the value of that cat seems to be inserted.
The s2_subscribed meta entry doesn't do anything as far as the actual mailings go, right? This is just what determines which checkboxes are pre-checked when someone views their subscriptions through the admin panel?
I manually built an entire form that basically does the same thing as the ?page=s2_userspage does in the admin panel, I just want to confirm that my suspicions about s2_cat* is correct before I commit the changes.