Besdies that fact that is completely illegal and I doubt anyone would ever get permission from Disney to do this, people will still try to satisfy their loved ones wishes.
That being said, be aware that I recently heard (on another Disney discussion board about this very thing) that inside the urn of "ashes" is an identification tag that, if found, can be led right back to the person to whom the remains were given. I'm not sure how big that tag is but I would ask the funeral director or the crematorium director before going ahead and dumping the entire contents of the urn.
WDW is probably one of the most beloved spots in the entire world. How many people do you suppose are already resting for eternity there? Makes you wonder the next time you're glancing on the ground near the Castle, eh?