A separate object is not needed, but a separate part. If you pick some poly's that belong to defined blocks (not half a block), randomly chosen, and give them a texture name, and pick some others and give them another name, and a third set, you can color those differently in TG, and it would look quite random.
You could mail me one as well, so I can have a go, if you can't fix it. But I'm sure you can!