Sumarizing, I have a TV which I rotate manually. I have a frontend with a horizontal and a vertical layout. I can change between both with a key, or with a sensor+script which does it automatically. You can change the bootscreens once the system is started. This can be automated by reading the frontend ini file. So, next time you restart your system, the boot screen will have the right orientation. However, if your have your screen set to horizontal, exit the frontend, shutdown, and then decide to rotate, next time you start your computer with vertical orientation you will see the bootscreen in horizontal orientation. To prevent that, you would have to run something before the boot screens appear, which, as we have said, it's impossible. That's all.