Just what the title asks.
Thanks in advance.
It has since 3.6.0 and it is called ZOrder, just like with GUI, and overlays are sorted among guis.
That's good for me. :-D
Since 3.6.0 manual is not ready yet, there's another bit of information:
Historically, there were 2 kinds of overlays: those which were always above GUI (normal speech, Display() messages) and thise always below GUI (background speech and overlays created in script).
For compatibility reasons, in 3.6.0 overlays created in script have a very large negative ZOrder (-2 billion...). This should not be relied upon, as it's just the way to put them below any GUIs and keep original behavior. If you are arranging overlays among gui or among themselves, it's better to ignore initial value, and set explicit zorder yourself to all of them.