Recently, we had set up an account at Nabble.com (see URL below) to
serve as an interface for the Mailman trial list:
http://www.nabble.com/Honyaku-f15518.html
Browsing around in the GG management area just now, I noticed that it
has a Remote Archive feature that does exactly the same thing (enabling
GG to serve as the Web interface/archive for a group hosted elsewhere),
so that might be another alternative to consider if Mailman is the host
ultimately chosen.
Best regards,
Sako