If SPDIF Out stops emitting clock signals in the absence of audio signals, would alsa_out working on the headphone jack of the USB soundcard that gets its clock signals from SPDIF Out emit pops and clicks?
Basically, the plan is
1. ALSA dmix(period size 512, periods 3) --> SPDIF Out of onboard soundcard(Realtek ALC887) --> SPDIF In of Creative Sound Blaster X-Fi HD USB --(alsa_in, period_size 512, periods 3)--> netjack1 ==(ethernet)==> ALSA jack backend --> Amplifier --> Speakers
2. netjack1 --(alsa_out, period_size 512, periods 3)--> headphones
I have my own software for managing jack connections. A keyboard shortcut can switch clients between speakers and headphones.