I used the aforementioned PC Audio Extension Bracket for my tests. Did not make any changes to it. Had to identify the pin assignments, pretty easy to do on this bracket.
As a caution, I would not use the 5volt(VCC) if Optical will not be used. Also, use one or the other(COAX or Optical), using both simultaneously could potentially overload GPIO Pin 13(signal(Vin)). Just speculation, but until otherwise known, why risk it.
Using the schematic Gouwa posted above, I assumed most SPDIF Optical ports would have a similar pin assignment. In this instance, the signal(Vin) pins for both COAX and Optical ports share a pin on the bracket, same for ground(GND) pin. For optical, the 5 volt(Vcc) is added.
NOTE: Brackets may vary, so users will need to use a multimeter to check for their specific bracket pin assignments.