The old mailing list software for @dragonflybsd.org mailing lists, bestserv, apparently allowed people not subscribed to a list to post to it, after answering a confirmation message for each message posted.

The closest way to duplicate that for Mailman is to sign up for the list you want, and then turn off mail delivery for your email address in the config page for that mailing list.  This won’t affect a lot of people, since most people want list output in their mailbox, but there’s at least a few I’ve fixed that way.