This thread might help you:
http://forum.unity3d.com/threads/dynamic-draw-order-or-2d-z-buffer.220231/
Specifically this post:
http://forum.unity3d.com/threads/dynamic-draw-order-or-2d-z-buffer.220231/#post-1475523
The idea is that if all of the sprites are in the same sorting layer and with the same 'order in layer' value you can use the "z" value of the sprites' transforms to determine which draw in front of one another. As part of your animation you can change the z values of the different sprites to make the arm move in front of the chest, for example.
I have not tested this myself but it seems that in theory it should achieve the effect you are looking for.
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