In short Mike, either you have to have enough thermal mass to qualify or enough insulation and thermal mass combined.
Unfortunately as you state though, you have an older building and it is highly unlikely that it could support all the weight required. It may need strengthening to handle the additional loads required.
Most firms that specialize in living or green roofs generally incorporate some sort of closed cell foam product to aid in the insulative ability of the roof and then specialized soil and drainage methods. Now whether they add the insulation above or below the existing sheathing or a combination thereof depends on some pretty specific site factors.
For more information:
Read "I am interested in converting the flat roof on my building into a green roof. I want to include raised planting beds for a vegetable garden." a Q&A answered by Richard Heller.