You can also paint this kind of map in substance painter: https://support.allegorithmic.com/documentation/display/SPDOC/Flow+Map+Painting
There seems to have an option in OX Mesh From Strands, to put normal of the geomtry in the direction of the flow instead of perpendicular of the geometry. But it look like it doesn't work.
With this option we will only need to rendrer the normal of the hairs to bake the directional map.
Is this option working or I am doing something wrong??