My recent experience with Porsche ownership has made me realise how good BMW are in including features.
When I was speccing my Boxster I found that multifunction steering wheel, rain sensing wipers, heated seats, 2 zone AC, parking sensors, and a load of other items were extras on a Porsche but standard on a BMW. Ridiculous difference in base price as well!
Anyway the rear view camera isn't standard on a BMW Z4 without the technology pack, and I don't think it can be specced on a new car on it's own. Similarly I doubt that it can be retrofitted but that's a question for the dealership.