It's not possible render CEGUI to Ogre texture in Ogre 2.0 (at least when using window render target as main rendering target).
Call draw() on GUIContext using texture target has no effect or creating artifacts on window render target. Even TextureTarget::clear() does not work.
OgreRenderTarget use Ogre::RenderSystem::_setViewport() in activate() method to switch rendering target between various textures and window target.
When build CEGUI for Ogre 2.0 this code is disabled via #ifndef CEGUI_USE_OGRE_COMPOSITOR2, so it's not possible switch target.
Restore support for viewports in CEGUI code for Ogre 2.0. Attached patch was tested in Ogre 1.9, 2.0 (window and texture targets) and 2.1 (only window target).