I had a few minutes free yesterday and I started to work on something similar. This thread triggered me into it, so thank you! :-)

I realized there's a big difficulty: all available grass comes in clumps. The one you used is one of the smallest but still not small enough. I think to have a convincing effect, it's necessary to use individual grass blades, not clumps. I will model a few grass blades and try it that way.

I'm using a 4th approach. In Zbrush, I started with a plane and designed the cobble stones there with a mask. Extruded the unmasked areas and so I was left with a plane with some extruded areas, the stones. My plan is to use an ecosystem with an altitude restriction so that the grass blades only grow on the plane base, not on the stones.

Unfortunetaly, because of my real life activities, I will only have time for this on the next weekend. :-(