its not possible with the mesh generator. Hand placing shouldnt take that much time, the place isnt that big. I think with the mesh generator you would spend just as much time cleaning roads and whatnot from grass. make sure the grass mesh is quite big though, and use ctrl to copy large chunks too when placing.
This. You can do this quickly, especially since the ground is flat.
Place one out, ctrl+D, pull it out, ctrl+D, pull it out- repeat until you have a good chunk of the thing, then area select a bunch, ctrl+D and pull those out and you' fill in the thing in no time.