I am sure that there is a fanbased site for Greyhawk somewhere... Try finding that and post your intentions there. I am equally sure that WotC would be monitering it and they may try cutting you off at the pass. This way they would be helping you figure out just what you CAN, and can't do with your project.

Another thing to think about is just how much referencing to the Greyhawk world you actually have in your work. If it is mostly inferred references, maybe it wouldn't be too hard to change the direct referenceing to infered and then I think that it would be okay to publish. But like mearin69 said, "I am not a layer".

Good luck with it.