What about an Index, layer 1, all the sprites inside it will have an auto-increased index, so, my last sprite will be the 1.14, where 1 is from the layer, 14 is from his Z order in that layer. This way, none of them will have the same index and you'll fine control them.
Sometimes I use Fireworks, Corel, etc, and there you can feel what the alspal is asking for. In Fireworks don't have the index, but have a list, where the top names are on the top of that specific layer, and top layer folders, where the top layer folder on the list is the top layer showing up.
What it means is the easy of use. Clicking, dragging and dropping a layer folder is much more easy and intuitive than any other method. And when you place your sprites inside that layer, they will be show inside that layer as a list.
So, I would suggest, why Our Layer List can't have a list listing all the sprites inside that layer, ordering them by his Z order, with unique Z order for each one?
Taking advantage of this situation, what about the UID being show by his side, in (UID)? Because it being hard to remember or check everytime the UID, but using this, you will make sure to have the correctly UID at least of the objects on the canvas.
More one suggestion, some drop-down menus by right clicking, like "blocking for edition" (that lock on the image), "hidden", etc.
Edited: After testing how to reproduce adventure games with C2, believe me, the Z ordering system can difficult it near to the impossible.
It's all because of the miss of an index where you can organize what is in the front and what is behind the player, who will walk around in a "perspective" World.
I tried to fix the origin point in the bottom, and check if it's down or up the player origin, and it worked, but when you send the sprite from behind, it hidden behind all other sprites too, but to make it perfect, I tried to organize it with and Code Indexed for each sprite. At least now, with 10 game objects on the Canvas, I have written near to 200 lines of code only to organize what is behind and what is on the front of the player.