Add the hair in the usual fashion.
By default the hair will just sprout wherever. To restrict it to a certain part of the mesh, create a vertex group in vertex paint mode, and then use the group you created as a density map (found at the bottom of the particle panel).
On the SAME LAYER (this is very important, I say because it completely stuffed me for a ridiculously, laughably long time), shift+A and add a curve (I ended up using nurbs as from my extremely quick experimentation it seems to animate the fastest, but any curve will do the job).