So just to give some information for future readers:
- I am pretty sure that my initial request is not possible, since the USB input will always override the S/PDIF input. Citation needed: I found a FAQ post in this forum, but can not recall where it is …
- One possible solution would have been to cut some lanes on the PCB thus that only S/PDIF input will be sent to the DAC. However, this is risky since one can not tell what the program of the XMOS will act in a such a scenario (+ it is a shame to butcher the PCB!)
- After reading Hardware volume - #128 by hoverdonkey multiple times, I came to the conclusion that I will do something similar like @occip and @ Coolheinze Which means writing a custom program for the XMOS, using their toolchain.
Wish me luck.