> To do this, wouldn’t you effectively need to make a graphics card (VGA would work) where a separate chip could read the screen buffer? And somehow get this card to display preferentially over the on-board video card?

If you do basic VGA (and UEFI), that'd be plenty for most. If it had a local output it'd be great for systems without video on the cpu (am4 non-apus, but also others)