Ok, I found a solution.
For those interested, it looks like this:
// class members:
BufferedImage photoImage;
FrameBuffer photoBuffer;
File photoFile;
...
// within some method:
// you might have to store the original camera here <--
photoBuffer.clear();
Camera photoCamera = new Camera();
photoCamera.setPosition(_from);
photoCamera.lookAt(_target);
world.setCameraTo(photoCamera);
world.renderScene(photoBuffer);
world.draw(photoBuffer);
photoBuffer.update();
photoBuffer.display(photoImage.getGraphics());
world.setCameraTo(cam.getJpctCamera()); // restore prev. cam
try{
ImageIO.write(photoImage, "png", photoFile);
}
catch (IOException ex){
...
}
As you can see, I have a dedicated (Software-) FrameBuffer for the purpose. I render into a BufferedImage object by getting its Graphics stuff.
There is no render-to-texture involved. The camera switching is a bit ugly but seems to work.
I'm not sure about the performance. But since it is only invoke once in a while I don't care so much.
Have fun!