Update: The plugin is ready. You can download the ZSorter.zip file containing the plugin and the sample project test_iso.zip from:
The plugin has one action ("Sort All Objects in Layer By Y") that you can call every tick. The action will sort all the objects of any kind in the layer that the ZSorter object is in. The objects are "Z sorted" by their Y position; if the Y positions is the same, they are sorted by their X position.
The sample project contains a layout with multiple objects being sorted in a layer. There is also a FPS count to determine how much impact the sorting has as the objects grow in number. I did not notice any impact in my system, but your mileage may vary. The sample also illustrates the technique of detecting collision with shadows instead of of the arpites themselves to allow sprites go "behind" a wall.The sample's walls are too small to notice the difference, but with if you are good at drawing you can make better walls and better shadows.
Please keep in mind that there is more to isometric games than sorting Z positions by Y. You need to take into account shadows (for collision detection), origin points (to allow players to walk behind walls, in the sample the Duck's origin is at the bottom and the wall's is at center) and more. Have fun!!