I think you would have to make the entire frame-window transparent and then add opaque child-widgets (panels) where you do
not want the transparency. You will probably also have to tweak the frame-window definition in the looknfeel (probably want to derive a new frame-window type for this) and make sure the titlebar and frame components of the window are opaque; this way only the
client area is transparent.
The other option is to use RTT and render the background + model onto the texture to give the illusion of a transparent sub-window.
p.s. I think Kulik was suggesting similar ideas, but it was kind of vague so I was not completely sure

If somebody helps you by replying to your thread,
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CEGUI.log and
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