For the record I think this is a great idea, in the past the wargaming community have rallied behind projects like Goblin Aid, Frothers runs extensive auctions and even gets sculptors to donate their work. But the key thing they have is support of the donators by explaining what they're raising it for.
If you want this to be a Dakka-centric thing, then you have a wonderful resource in the forum itself to help choose a charity. Run a poll, or make a suggestion topic, find out a couple of charities people here care passionately for.
Asking a studio or individual to give up several hours of their time (effectively wages), and then pick the charity is a nice idea, but you may need something a little concrete to convince them to do so. Having the backing of a large, passionate community to show that time and effort will be cared about would help immensely.
Think of it in another way:
Someone turned up at your door, as they are apt to do, and said, "Donate to charity."
"Okay, which one?"
"You decide."
You'd wonder a little about what to do, or how to respond.
Whereas if someone turned up and said, "I'm here to raise money for a charity that hundreds or Wargamers you know are very passionate about, would you help?" You already show how important the idea is to other people.