So I can modify my SNES, GCN, Saturn, and Dreamcast for S/PDIF. My PS2, Xbox, 360, and PS3 all have S/PDIF out of the box. Why can't I have S/PDIF for my Wii? I googled it, and nobody seems to have come up with an idea for a mod. So I'd like to talk about it here.
For what I was able to deduct from experience, the original specification did never intended to support digital audio out on the Wii. As technology advance, functionnalities are embedded togheter inside single chips. Then audio decoding on the Wii is probably done by a DSP by fetching data straight from memory.
The DSP is most likely containing the required hardware to internally convert the data stream to an analog stereo signal. Since this design technique is most likely used to reduce cost it is likely that in the same fashion the required data stream is not available on any pin of the DSP IC.
If you want to go further you'll need to locate the DSP responsible for decoding audio data (most likely connected via a bus to the CPU) and get a pinout of this particuliar chip. Then you could get a definitive answaer wether a digital audio mod could be easily installed. If it is not the case, then the only other method I see would be to intercept the audio data sent from the CPU to the DSP and decode it yourself to produce an SPDIF audio stream.