The keyboard's plists of PressAndHold also can no longer be modified to add the new accents in El Capitan
Files in /System are protected by System Integrity Protection. If you wish to edit such files, you will need to disable this.


  To disable System Integrity Protection, you can do the following:
  
  
  Boot into the Recovery HD by restarting whilst holding ⌘R.
  Open Terminal (from the Utilities menu).
  Run the following command in Terminal:

csrutil disable

  Restart.
  


Source: Disable OS X El Capitan Rootless and permit write actions to System Integrity Protection locations

You can re-enable it once you've edited the file.