Having looked at this a few times, I think that the best place to do it is actually in a G-code filter. ie calculate the heading from the G-code and insert / append C axis moves as appropriate.
Actually, there is a use case that needs motion pins. A plasma cutter cutting bevels. An additional joint is needed to tilt the torch and there are complicated tilting torch holders that keep the torch tip axial to the Z axis (single axis bevel head). I was just chatting to a user with one who uses the post processor approach.
But picture a simpler torch holder design where the torch tip position moves when tilted. A component could take the bevel angle and calculate the offset x,y,z positions of the tilted torch tip and then apply external offsets to locate the torch on the cut path in real time. This way, the machine could consume simple 2D x,y gcode (which is the norm for plasma cutters) and enter plasma cutting nirvana with bevelled holes.
