Hmmm. Obviously, you know I'm huge into SPN, but the idea of an Organization XIII cloak really has me on. There are a fair amount of SPN items, already - and as much as I adore the ideas you have listed, I think, for the most part, there are decent enough alternatives for them, whereas I can't really think of any good Org XIII coats.
The forum event could be collecting hearts to power an analogue of Kingdom Hearts! Depending on the scale of the event, the NPC could be a Xemnas- or Xehanort- or even Ansem Seeker of Darkness-like Corenaught (instead of 'Nobody') summoning Sansheart (instead of "Heartless", from
Keytarblade) for you to defeat. "Kingdom" could be "Dominion" or "Monarchy" or something? You get the item as being welcomed into their ranks when you hit the max amount of hearts collected. As to the manner of collection... Maybe the quotebots post around in threads, and however many you have in yours every hour is how you get them? Keep an active thread in the appropriate forum and you're sure to get a bunch?
Or, theme-wise for your idea... how cool would it be to have some sort of avatar change for the people converted? Like they had with the magic school creatures thingie mini-event a year or two ago, where being converted changes how your avatar displays unitl the event is over or you get it changed back?