Actually a good point.
My approach for this would be, to move the camera (head) to the area where the QR-code should be and move the bed up and let the cam “listen” for the code. As I assume that the camera has no auto focus, there must be a defined distance, where the code is in focus of the cam. If the code is detected, next is the determination whether or not the correct plate is installed. If the nozzle get’s a “hit” (like during probing) before the cam detects a code, it must be that no plate is installed.
I’m no programmer and have no idea how this could be implemented. But I’m hoping somebody with the skills is reading this. 
However, I’m sure something like this detection would be very welcomed in print-farms.