The dynamic hair doesn’t use alphas at all. In the dynamic hair shader with the length and friz, an alpha map will be read like height or bump map. Dynamic hair does not have or use an alpha, it is individual strands of hair, so no need of an alpha.. These are all done with gray scale maps, not a duo tone alpha map, The hair bump map is more useful in those channels the an alpha map. An alpha is on or off, black or white, it takes a gray scale map; like a bump map, to set hair of different lengths, thickness{hairs per area} curl kink wave and so on. Yes, you can do a mullet with a distribution map in the hair length.
I played around with this for a bit, logically you could use distribution maps to set your hair style, but I never could quite figure it out.