Child's Play seems to be the go-to charity for gamers, nothing else off the top of my head.
Tracking specific things in-game such as airshots seems messy to me, but of course I have never hosted something like this or anything, so I'm not speaking from experience. But I do think maybe a little round robin tourney of sorts may be a good idea, and people could pledge donations for wins or for one team to win, whatever.
I'm sure you've thought of this already, but looking at past/current other charity gaming events would be a good way to gather ideas.
I can't really contribute much, except maybe art for ads/Twitch banners or something. (Fiddled with Boink's Twitch for example. Aaaand a larger version of the banner.)