ParticleInstance produces particles with orientations that are arbitrary (the orientations are currently determined by the geometry of a tessellated mesh which is automatically generated and which the user has no control over). To get around this, I added the ability for the user to choose a UV set from the object emitting particles that would be used to provide an orientation for each hair particle (ie, the starting normal and binormal). This allows the user to easily produce particles that line up and form natural looking grains.

(on the left is current behavior where the flat hair particles are running in different directions. On the right the same particles are using the uvMap of Suzanne for alignment).

I've also added a field to the ParticleInstance UI that will allow the user to select a UV map from the object which emits the hair particles.