There have been discussions on it on every Guzzi forum I know, including here:
http://www.v11lemans.com/forums/index.php?showtopic=3381
http://www.v11lemans.com/forums/index.php?showtopic=4315
I only know the Ago helmet, first done by AGV, later by ARAI. X-lite had one too, only one year production I believe
BTW, anyone has experience with M2R helmets (US built I believe)