Hi !
This is an amazing new option in Forest Pack Pro (beta version 4.3.2b) but it's, for the moment, only available for VRay.
It's an easy way, just in changing the boundary checking (in Areas Tab) from Point to Edge, to have grass, flowers etc. in very small areas, or simply in straight edges, like I did here.
I really really hope to have this option available for Corona as soon as possible.
Here is the topic I had with Itoo developers :
http://forum.itoosoft.com/index.php?topic=2604.0