I am integrating a single osgQt::GLWidget in a Qframe and using the pattern here: http://trac.osgeo.org/ossim/browser/trunk/ossimPlanetQt/src/gui/ossimPlanetQtMainWindow.cpp to switch between full-screen mode and back. Basically consuming the keystrokes within the widget and setting full screen from there.
I also have an event consumer in osgViewer::Viewer - osgViewer::WindowSizeHandler which handles the viewer size switch.
Is there a better pattern to implement this ?