Sorry to pull Cryengine out of my hat again, I'm not a fanbox or so, hehe.
But you have an additional checkbox near every terrain layer to choose which vegetation type will auto-grow there.
This would be a nice thing to have. The tool uses the values you entered in the vegetation type like distributin, lod ranges, etc..
A texture layer can have multiple vegetation specimen set.
Downside: While using it for wide and open fields I noticed the vegetation was popping out of nowhere on an entire terrain "quad". It's hard to explain but it looks like if it doesnt blend in the vegetation with its viewdistance but all in one on a tile.
Maybe Josh can do better.