Thanks, Claude!
The stitching process is really not the problem, as all this is already implemented in Extended View Toolkit [1]. My problem is more the computational effort, that is multiplied by 3 when using 3 virtual cameras. Plus the stitching part.
Perhaps I need to explain more what I want to do: I have a very complex 3d scenery inside GEM. Now I want to view this scenery from the center and then project the image onto an immersive panoramic video screen. The screen is not a full dome, but more a cylinder surrounding the visitor. So the virtual camera inside GEM ideally also covers a cylindrical area.