You could get a conforming cape by grouping the top part for the chest and perhaps the shoulders. Additional bones for cape movement wouldn't matter for purposes of conforming if done properly.

I assume you know how to create clothing by loading the target figure's skeleton and deleting unneeded bones. 

I would minimize the cape skeleton complexity while making a ton of full body morphs for the cape and adjustment morphs for areas like the shoulders and upper sides. 

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