func_wall is still a brush and still has its own texture from a wad file... models themselves are what you'd see in hlmv...
as far as I know.. theres no way to change the actual texture used, you *might* be able to edit the textures listed in the bsp file, but that would require a download becuase textures are loaded out of the client side bsp..
also changing a model texture thru hlmv, the client would have to download the new model to be able to see the texture..