I was told that part of the reason the Ammonia's are or were not on their short term radar was because of the lack of details they have to model the underbody of the wagons. They don't want to do a Hornby job with a blank barren underside and at the same time they don't want to make up an educated guess at the detail and model it either. That may all have changed but that was the case I was told, it's that they want to do it right or not at all.
My view is, if they do make a guess at the underbody and no one has any info to help then no one can correct them or critique the model if and when it's released.