I'm not sure what you're after regarding Z order. Construct has various features to control Z order, such as send to front/back, order relative to another object, and sorting Z order by things like instance variables. Which would be relevant to your addon?
AFAIK the Addon SDK docs are up-to-date so if they don't cover something it would need to be added.
Ashley, ordering relative to another object via code would be perfect actually. Especially if I can process multiple objects in a row. For instance, I can take object1, place it above object0, and then take object2 and place it above object1 (above the new z order that was just assigned to object1). Being able to do this with several objects within a single tick without having to wait for a z-order update would be great. (and without a significant performance hit from forcing multiple full z-order updates in one tick)