Connecting USB controller to I2S inputs on DAC+ DSP

I'm working on a project that I'm considering using the DAC+ DSP for that would pass digital audio into the pi for processing and then back out, using the DSP probably only for sample rate conversion.

However, I'm also curious to add a USB controller to the DAC+ DSP via the additional I2S inputs, so that essentially the unit could act as a USB audio interface.  I'm looking a boards like this to get started.  I'm new to this, so a couple questions:

Would it be a seriously complex operation to add in one of these XMOS chips or something similar via the additional I2S inputs to the DSP and then be able switch between toslink and USB? 

Would it be much simpler to just configure the USB chip to output S/PDIF and then use maybe a hardware switch to select between them?  Obviously would have to modify the board to do this, which is why I'm particularly curious about those I2S inputs.

If anyone out there has tried passing USB audio into this DAC+ DSP, I would love to hear how!  



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