I just read your QQH. Sorry, so late. I don’t know how to handle QQH(it jump out itself, after I left that place, I don't know how to go there again), so respond you here.
Read your case, I don’t think I really have experience in that aspect. Just help thinking together: it’s nothing but two points: (1) The conference is very great. The point is “great”. What you can do is to find something to elevate that “great” to “extreme great”. (2) You are an import person in that conference. The same thing, to find something to elevate that “important” to “extreme important”. Anyhow, “I am greater than others, so I have extraordinary ability.”
If you can find a big guy to say the above words instead of himself, that will be completely different.
The following is the communication I did with another person, which you inquired.
The first to him/her:
I got help from the lawyer. So not really ever used my own head to think too much. Recall asking proof from the editors for my review work, I only asked them to respond my e-mail a "yes" for the papers I listed, and then print out. But I collect the background of each journal to prove that all journals are the first in the world (kidding). Though kidding, I think I made great effort to prove the excellence of each journal.
The second to him/her:
The letter should prove
1. The fact that you were chairman.
2. How important that meeting was.
I used my example to suggest the above to you.
Good luck.