I can think of a few concept cars that had such devices, like the Audi Rosemeyer, or the 2007 Nissan skyline coupe... here a pic of the Rosemeyer sportwagon:
You can see the rearward looking cameras are in the two bulges on the roof. Don't now where's the screen though.
I'd say the only reason why it hasn't been implemented is because of current homologations in EU, USA and Asia making mirrors mandatory.
I'd say the better place to put the screen(s) would be at the same place the Peugeot 905 did put it's regular mirrors
(ie in board, on both sides)
As for racing, I'm pretty sure a few GT car like the saleens were using camera too
(along regular mirrors) to deal with the prototype's random overtakes.
EDIT: oups, reading your initial post again, I realise you were speaking exclusively about that for a formula 1 car. Sorry. Placing the screen would be tough indeed!