Well, the thing with Blue Gramma, you will need at least a deep semi-intinsive, if not a full blown intensive green roof. Prarie grasses, which Blue Grama is, have deep roots to hunt for water. It is heavy soil because it needs organics, which hold water. You need to have a support to act as mini retaining walls, so there will be “fins” that extend vertically from the roof. There are tons of different ways to do it, and not knowing the details of the install, its kind of hard to comment it.
I know you said you didn’t like plant pockets, but this seems like a good system for what you want to do .
http://celltekdirect.com/gallery-photos.cfm?c=5D677E32-3048-71C2-1756E3655FB6A636