For some reason when I use a texture overlay on non-portalable walls (or maybe it's just the texture itself), it either doesn't appear or that part of the wall is just black. Why does it do this? Am I forgetting something? If it is not possible to texture this way, how else could I go about doing it?

Remember not to be putting it on the non-portal entity if your using that.
I think that most non-portal textures use bump-map texturing so i don't know if it'll work.