ooo good idea! I have cypress usb that we used to develop high end audio player, i think it has I2S and we already have audio driver for it :) I will ask my guy to see if we can hook up 2 things in one :)
There are some I2S to USB converter around, not sure of the cost, probably expensive though. TAS1020B chips definately does I2S to USB. You can use 2 SPI's from your main MCU to rip the I2S stream and convert it to WAV, not hard to do but I guess it would be too much work just for a demo.