My first two thoughts are:
You might be able to override the reshape (that's what it is in GLUT, search for the call that pushes a resizeWindow event on the OSG event queue for your particular window system) - always pass it the full width/height of your screen instead of your window, and tell it to use 0,0 as origin, or alternatively just override it with an empty function if you'll always start with a full screen window.
If that doesn't work, you can reset your camera's view frustum based on your window's dimensions and your previous knowledge of the full screen dimensions. There should be lots of info out there on changing your projection matrix - if you can't find OSG-specific info, any OpenGL info about the projection matrix is valid for OSG as well.