System Integrity Protection — Is it implemented in Darwin? Or is it OS X only?
SIP is exposed to the file handler code as a new "restricted" class flag ACL as if it were stored in the file system metadata, so the code required to prevent all writes would be fairly transparent to implement so that all other packages would just get an enoperm error if my suspicions are correct. 

You are correct that we don't have a source dump yet for 10.11 so I reserve the right to be totally wrong about my suspicions.