> If you've got an experiment (either relativistic or quantum, or both) where two individuals can obtain different photon counts, I'd like to hear about it.

The experiment could simply be measuring photon counts at a pre-agreed location after a pre-agreed duration, where the elapsed time is measured by a local clock. (Probably it is reasonable to argue that this is not a "reasonable" experiment, but it's beyond me to make rigorous the sense in which it should be excluded.)