Shadow baking. For hair texture.

I am trying to make game style head. I have make a planes with alpha that I rendered with blender cycles but still. Scalp is totally bald.
Why? When I draw hair by hand they look not so nice and I cannot find nice texture or picture to projection painting.
I found this :
I think if i will somehow bake shadow from my hair pains then head will be not so empty.
I can bake solid objects but not planes with alpha on it… or paticles… And when I make mesh from hair partilces and render it render is very pixelized.
Halp ?