What you want to do can be done, but the way you describe will not work. Go to www.johnbridge.com and scope it out. What you want is called a barrier free shower. Essentially, the whole floor needs to be treated as a wet room, and this requires a liner, either surface or subsurface. Check out www.schluter.com for Kerdi. There are other methods that would work, but what you are thinking is a disaster waiting to happen. From any area that can get wet needs a constant slope of 1/4" per foot to the drain. Hardiebacker is not waterproof, and after you installed it like you indicated, you'd have all sorts of holes. Plus, any cbu must be installed with a thinset under it. The tar paper would give you all sorts of grief. It could be used on the walls, but not below about 3" from the floor or curb, where you must have a waterproof liner.
Unless you can recess the shower area of the floor, your floor will end up quite a bit thicker than the hallway leading to it to account for the required slope to the drain and the thickness of the tile.