I'm hacking on matchbox-keyboard. Its UI is written in plain x11. I just want it to be always on top because it is a virtual keyboard; a virtual keyboard can't just get below programs! I don't want a WM-specific solution because it should run anywhere.
So how does the other virtual keyboards stay on top?
I did try the _NET_WM_STATE_ABOVE thing. but it didn't work (at least on unity).
You probably need to set the extended window manager hint _NET_WM_STATE_ABOVE.
In general, if you want to copy X behavior of other applications, you can use xprop (in my distribution packaged as xorg-xprop, YMMV). When launched from terminal, you can click on an application of which you want to see the window manager hints and properties. This can be of great help to steal tricks.
_NET_WM_STATE_ABOVEdoesn't seem to work on Raspbian / Openbox or XFCE either.