Hi Lawrie,
The foliage tool does not have movable objects. This is why it would not let you paint on the movable object. Even with the workaround to paint on it, as soon as you changed the box from static back to movable it detached the foliage from your box.
You can place the grass meshes on your box manually and list them as movable to achieve this action though.
Let me know if you have any other questions. 
Tim