It would be easy to write a relay for the list, that wraps all received messages in kind:6 reposts and tagging them with sequence numbers. It would serve only reposts signed by the relay, the relay could be fully replicated using strfry sync. It’s actually quite trivial. The relay would only serve and store the wrapped messages.

PGP has one key only as well, a mailing list ist in no way more secure than a nostr message.