Thanks mostly to Frank B, we now have direct S/PDIF output from the Teensy Audio Library!
Here's a detailed blog article.
You can get optical TOSLINK digital audio with a $1 connector (Digikey 1080-1434-ND) or just a red LED shining into the optical cable.
In the photo above, the TOSLINK connector is mounted on this OSH Park PCB (under $2 for three of them).
The S/PDIF output is also now supported by the Audio System Design Tool, so you can just connect it to whatever complex audio system you like, to get S/PDIF digital output!
Here's the lengthy forum thread where this development was discussed.