I think the success of this concept depends on how frankly they're willing to approach the racism angle. Obviously there are certain things you're not supposed to say even in unflattering portrayals of racist characters, and certain things that Will Ferrell probably doesn't want to say for the sake of his career. (Side note: I find it a bit curious that period movies can get away with patently offensive displays of racism while movies set in the present tend to shy away, as if we're all pretending that we've solved the racism problem. Any counterexamples? Monster's Ball, maybe.) There's a lot of potential for trenchant satire in something like this, and a lot of potential to make something anodyne and boring, too.
No word on what comes first, this or Sherlock Holmes.