Mechanisms for linking together multiple lists, whether or not they
     are all using Mailman, are fairly well understood.

   Well, I don't know them =)  I looked into mailman and found it reviews
   the existing List-Post header (which I learned is a normalisation
   upgrade from X-Loop and x-BeenThere) to prevent mail loops, so I'm
   considering that one could simply subscribe the lists to each other and
   mirroring would run fine.

     In the case of Cypherpunks, the CDR description you found worked
     pretty well, as I recall (as a subscriber, not a list administrator
     at that time).
     If you are keen to restart remailers for Cypherpunks, probably
     you'll need to give thought to who, what, where, why? ("How" is
     important, but not that hard.)

   Seems these are important things to do even when you're not sure of the
   reason.
   I'm guessing the missing bit would be to automatically subscribe to a
   peer list during setup, so people can configure them swiftly.
   It would be nice to have signed digests of preceding list mails in a
   header somewhere to make it clear when mail is dropped by a connection
   issue.  It would also be nice to have permanent storage of archives.
   With more people running servers more people can possibly implement
   such things.
   Similarly with variety of location, other technologies, etc.