[SDPhantom]

The best way I can find currently is use GetRaceAtlas() that is provided for the character creation screen. The atlas name generated can be applied using texture:SetAtlas().

Here's an example:

hooksecurefunc("UnitFramePortrait_Update",function(self)
    if self.portrait then
        if UnitIsPlayer(self.unit) then
            self.portrait:SetAtlas(GetRaceAtlas(
                select(2,UnitRace(self.unit)):lower()
                ,UnitSex(self.unit)==Enum.Unitsex.Male and "male" or "female"
                ,true
            ));
        else
            self.portrait:SetTexCoord(0,1,0,1);
        end
    end
end);