Could you pop a washer in there just to raise the caliper a fraction?Nope.
I put Hope rotors on mine and wanted to go 220/220 so contacted Canyon and they advised the frame is only rated to 200mm so any issue would not be under warranty. Shame because the 200mm Hope rotors slightly rub on Shimano brakes but the 220mm up front doesn't.
Does anyone do a thicker rotor? I suspect that would be more useful than a larger one on the rear.