I believe that even without tampering with system files, you are allowed to change the region code of your Mac's DVD player app 5 times. After that, it becomes permanent.
I'm pretty sure that if you insert a disc with a different region code than you've played before, you'll be asked if you want to change it, then warned about how many changes you have left.
I have no idea where or how these region codes are stored. They may well be in a PROM chip in the drive. I know of no way to increase the number of region changes permitted.