You and others here probably already know this trick, but you might be able to get automatic echoes if you use the list address as the moderator of the newsgroup, such that replies are forwarded to the moderator address rather than being posted. Might also need to massage the outbound message so that it strips off unwanted NNTP headers.

A bidirectional mail/news gateway is lots harder than what I use for NANOG. In addition to the above concerns, there's a need for the "mail side" to keep its own message-id history for round-the-horn duplicate suppression. This is how the namedroppers@ops.ietf.org <=> comp.protocols.dns.std gateway works, for example. (The news -> mail direction is moderated, first by ISC and then by Randy as are all postings to namedroppers.) However, for my private NANOG newsgroup I prefer that followups just silently die. Any local user of my nnrp server who can't figure out that they need to use e-mail to post to NANOG, probably ought not be posting to NANOG :-).