John,
Having lost my brother, who was also my best friend, 2 years ago, I became aware that the words of condolence were of less impact than the fact that the person was impacted by the life of my brother and took the time or effort to acknowledge that in any variety of ways. Just the handshake or the hug helped lift and take with them a bit of the sorrow that we felt.
I think that about the only thing you can't say is "I know how you feel" because we never can know what another person is really enduring.
For me the gradual transition in the grieving process is one of moving from the total sense of loss to now being able to remember more of the good times.
I feel that just letting his family and loved ones know, in the manner that you outlined, that he had an impact on us as individuals and that he was one of the treasures of this forum may act as a virtual group hug from all of us.
Thank you for your efforts in co-ordinating this project.