Dynamically create Texture2D that is readable

I want to create a Texture2D dynamically which is readable. There doesn't appear to be any way to set the IsReadable property to true. The docs reference IsReadable, but I can't even find it declared anywhere. I have

Texture2D tex = new Texture2D(width, height, TextureFormat.RGB24, false);

The size of the Texture is runtime dependent, and so I can't create it using a TextureImporter. I also can't clone an existing Texture2D which is readable because I need to change the size.

Thanks