It is possible to set the headland to one circuit, do one pass on south end and  east side, setting a and b on the way, go back to select pattern screen and change it to 3 circuits, do north end and one pass on west side and remainder of south end, then set it back to 1 circuit and select your a-b line and commence planting.  This allows you to use the a-b line on pass 2 and 3 instead of having them based on the crooked outside pass. 

They should make this possible without so much monkey business.  Who wants to plant three crooked passes all the way around the field??