Hey!
I’m not sure if this is what you are looking for, but I stumbled upon this a couple of days ago. You can take a look if you want. Wind Shake: High Performance Wind Effect For Leaves & Foliage (by boatbomber)
Hopefully this is of use to you. 
Edit: Just re-read the question, if you were looking for a way to add ‘still’ grass effectively I would agree with @budmomo