An alternative to adding a Maildir backend to a news client would be to produce a news server that's backed by Maildirs. I could thus get procmail to deliver straight into groups on my virtual news server.
Posting is an interesting problem: different mailing lists have different conventions (whether to reply to all or to the list, for instance). This would probably require some manual configuration for each list.
Gmane provides similar facilities, although they are database-backed.
The inverse of this idea — an NNTP-backed IMAP server — is also interesting.