I've written a short article on an alternative way to z-sort in the context of an isometric viewpoint. I don't know if such a solution has been presented before, so pardon me if this is old news. Most of the sorting solutions I have seen is based around the Y-position of Sprites, and I had a specific problem that led to this method.
The purpose of the alternative is to enable sorting on Sprites which have an 'irregular' shape, which means the image goes beyond the usual square/diamond tile area.
This is just a starting concept with simple examples to illustrate the idea, so don't know how much modification it will have down the line.