Hi. Thanks for the challenge. We got to work, and Paul came up with the best solution first.
This method uses combination of techniques.
First we use Forest Pack with exclude areas, but additionally add two effects- One for scale, and one for repulsion:
These are controllable with curve, and numeric entries. This might be enough for what You wanted.
But Paul went one step forward. Here is technique, how to achieve more organic shape, as in Your reference:
There are few modifiers on top of forest object that does the job.
And for the grass You create new grass object and add tiles object as exclude area. You will have to play to get acceptable parameters, but ForestPack does the job pretty well.
Attached scene for Your reference.
Hope this helps. If You got any questions- ask.