One alternative not mentioned: use of TOPICS (up to 23).  This requires
ongoing list-owner and poster vigilence.  Useful for lists that don't
have a high subscriber churn so that established posters can figure out
the semantics.

Partitioning your lists, and subscribing existing folks to them is not
that big a "spam" issue in my mind.  You could certainly start new lists
(temporaily SEND= OWNER) and  give folks a grace period to subscribe
themselves (the old list could have sub-lists to the new ones).  Then
after the grace period, remove the old list, and update the new list
definitions to SEND= PRIVATE.

Postings could have a bottom_banner appended with a URL to the "rationale."

Whatever you do (or don't), you know that you're not going to please everyone.

Pete Weiss @ Penn State