Is it even possible? If you hover over an empty container it will be marked as Empty but with a diamond symbol before it. How that symbol is made?

The symbol can be found inside the Hudmenu.gfx...or was it .swf? However, it's inside the Hudmenu flash file.

Modifiyng the symbol could be done with S.W.I.X and Trilix, or you'll need to learn flash...which probably leads you into one of the Sky UI team threads.