Sorry if I interfere,
maybe I am wrong, but I can't see a problem here.
I would do it like this. ( lets say we take the A axis )
calculate the steps per so that you get 1 Rotation of your spur Axis by 360 degrees.
UNCHECK "A axis is angular" in Gen. Config UNCHECK rotational options.
Now a code like G0 ( or G1 ) A180 should turn your axis 180 degree in one direction
G0 A0 back to old position.
Now IF there is a problem with the rotational axis ( A , B , C ) ( no Probs here )
THEN you can use the X or Y or Z axis for that, IF you have any as a spare.
Same setup ( steps per unit etc. like A,B,C )
just a thought
Alex