Hi DSutherland.
The Prodedural Grass System only supports spawning grass meshes on landscapes. In your case I believe you can use Foliage Painting to place the grass meshes over the static meshes you need.
A fully automated solution could be possible via a custom construction script. You can trace static meshes and check for the Physical Materials on these and use the output to filter where to spawn Hierarchical Static Meshes over the surface of the static mesh.
Hope it helps.