keltheos said:
Well, when you have to get material approved by GW, then their current preferences have to define what you're writing. Ork Freebooterz in Rogue Trader can't gain Corruption because GW said so. You may disagree as vocally as you like, but that's the current direction of the official background, and thus you won't gain any official material supporting the idea of Chaos Orks.
And honestly, I agree with it. Having a Chaos version of everything else just feels tacky, and denies Chaos a defining 'feel' of its own beyond being "X with spikes and daemons". It also runs into the problem of "it's all the fault of Chaos!", which takes something away from the setting as a whole.