The one kink in Fox's armor, from a legal perspective, is why did they wait until the film is on the verge of release before going to WB about this? SOMEONE got word back to Fox early on that the film was moving forward. This is Hollywood we're talking about...the in-breedingt that exists there is the entertaniment industry equivalent of the kid on the porch in Deliverance.
This is just Fox's way of making some fast easy money. They're hoping to strong-arm WB into either paying them outright or giving them a cut of the take. Don't think for a minute that artistic integrity or any of that happy horse shit has anything to do with this.