This is the only picture I can find of the original building with a proposed grass roof. I've circled it. I can't recall any talk of a green roof on either the south or north stands. The picture also shows that indeed these roofs were not going to be "green".
I got this picture from...
If I remember correctly, the green roof would have been on the little annex building in the corner of Bennett's field where the proposed hotel would have been. It would have been accessed by a glass walk-way from the actual stadium. There are pictures of the plans for this somewhere but can't...