I'd point out that Google already provides an excellent (free) search
facility for the MMD archives. In the Google search box simply use the
site delimiter; for example, in the Google search engine window type

  site:www.mmdigest.com diamond jubilee

and it nicely returns all the references to my organ (also a useful
trick to search Craigslist nationwide for obscure items).  There's
probably a clever way to implement the site-specific search as a link
on the MMDigest page.

On the subject of a forum, I think it would be a nice addition.
I don't think anyone has proposed replacing the Digest with a bulletin
board (nor should they!); they perform different, complementary
functions and could happily co-exist.

Cheers,
Roger Wiegand
http://www.carouselorgan.com/