I don't know that it's possible using a single implementation. There are several that all take a different approach.
My personal approach would be to take a look at how paul added grass. His implementation takes a mesh and decides based on the angle whether or not to place any grass, but you can add your own too. I added a noise function to it to create areas where there was none or very little grass. It made for nice "paths" to walk through. You could use something similar to "paint" the grass - maybe using an array of some sort.
The whole class I linked to is important, but the line I linked to is where the decision is made as to whether or not to place a grass blade there.