I want to add a battery widget to the systray. How can I do that? How does systray recognize what programs to put in it?
It just occurred to me that this question has the awesome-wm tag, so ignore my X11-answer. Here is another attempt.
There is no way to add anything to a wibox.widget.systray from Lua code. However, you could just put a awful.widget.imagebox or awful.widget.button next to the systray in your wibar. I think that having another widget next to the systray like this should look just as if there was another entry in the systray.