It looks like Cocos2D offers an abstraction layer on top of openGL by allowing the developer to set the depth via zOrder or vertexZ on a per node basis. This breaks down for large than 1x1 nodes (e.g. 1x2, 2x1, 2x2) that span multiple isometric tiles, as multiple vertexZ values are needed for different parts of the node.
One solution to this is breaking the node up into 1x1 nodes each with their own vertexZ value. This is fairly cumbersome and requires additional logic and manually slicing up the sprite. The alternative solution, in openGL, seems to be running a depth test on each pixel using a depth buffer. Is this possible to do within cocos2D? or will it require a different rendering path completely?