All serious cameras do - but it's typically fairly small and most image review happens from the sd/cf card, not the buffer. I think if you have instant review on (I don't ever use this setting) then it MIGHT happen from buffer, but I'm not certain.

Anyway - yes on the face of it this seems possible - but most pros I know will shoot a large chunk of images and review afterwards in a batch - they typically don't review often enough for this to really work, otherwise they'd risk missing shots while reviewing, which is why I thought that in practice it might be hard to implement this well enough without compromising security.