Did you ever solve this? The problem seems to be related to the fact that there is both [MTex (Material)] and [MTex (World)], they are not interchangeable. The docs do not cover MTex for World. The only way I see getting it to work is if there is already a Texture object applied to a World and you retrieve it and manipulate it. Having to use the UI to pre-create some World objects with textures does not seem to be an ideal situation.