David Andrews writes:
Thanks everyone. This all sounds reasonable except it also identifies another problem we are having. There is no good search method in place.
The only way to resolve it in Mailman is to upgrade to Mailman 3, and use HyperKitty. As far as I know you can import a Mailman 2 mbox into HyperKitty without installing the rest of Mailman 3. If you decide to go that way, don't use the default indexer Whoosh, it's tragically slow. Xapian is highly recommended, and other indexers also seem to work. I think I've heard of ElasticSearch installations, and one friend used freeWAIS -- that's mostly a joke, freeWAIS is unsupported for decades, but librarians/archivists might feel some nostalgia.
Also the organizations librarian wants the archives for historic and other reasons, so leaving them here won't satisfy that.
As Mark says, there's a $LIST.mbox file at the top of the list archive subtree for the list that contains all of the posts, which most all open source mail clients will happily treat as a mail folder, and I would hope commercial clients do as well. Just download that to somewhere the mail client will recognize it as a folder, then you/they can use the client's capabilities to organize it.