[brazen1]

Yes.  A custom mode can be set as long as the mode you are setting is not the active mode at the time.  You don't always need to play a video since you are in 60Hz desktop vs 24Hz assuming 24Hz is the mode you want to set.  I do it like this all the time.  However, when you want to force 12bit to be compatible with a custom mode, you have to be in 12bit.  Otherwise, your new custom mode will only play at 8bit period.  The only way to enable 12bit video mode and still retain 8bit for the desktop mode is to play a title and then change the refresh rate to 30Hz or something since you can't create a custom mode when that mode is active assuming it's 24Hz.  Then you will get 12bit with a custom mode.  But considering 12bit might introduce banding depending if your display is 10bit or 12bit native even after dithering down, it may be moot.