So I see that MailArc can archive messages from a mail list - can it go the opposite way, so that users that only want to use the forums can post, and the mail list users can see and reply to it.
It shouldn't be too hard to add code to a) have a user that signed up to the forum be added to the mail list subscriber list - but not to recieve email. b) have any post be bundled up as an email From: the forum user To: the mail list. - Is that by your estimate an afternoon's work?
The first part depends on what you are using for the mailing list, as it would require modifications to that as well. Sending out a mail to the mailing list with the proper headers would be quite easy if you are comfortable in perl.
If you want to do it as a plugin that will survive upgrades, you'll want to look at the insert_post hook. If you just want to modify the code directly, try GForum/Table/Post.pm, sub _plg_write.