Wediboard comes in various thicknesses, and if you choose one approved for use on a wall, you should be okay. Thinsetting the tile to it stiffens up the assembly considerably, but you still need to start with a reasonable thickness. You wouldn't want their thinnest board, unsupported on a wall. You'd want it to lap over the tiling flange. You could notch the back of it so that it lays flat over the flange.