playing a continuous sound file will be a bit annoying at times, as I do a lot of recording audio from my desktop, and even if it isn't audible, the sound waves will be visible in the recording program.
aplay /dev/zero
only sends samples with the value of zero
so it won't show up in your recording program nor add anything to the signal. It has no side effects besides keeping your S/PDIF alive (if it works for you ofcourse).
b.t.w. On my multimedia system I let the route to the optical out bypass pulseaudio, sending the output of mpv and audacious directly to the optical out in either S/PDIF, DTS or AC-3.
Only in very special circumstances I switch to pulseaudio either optical or analog out.
On my home PC I have pulseaudio over both S/PDIF and analog out simultaneously.
On my recording PC I use an UMC1820 over USB
P.S. There is a tiny chance to get hiccups in your audio when using aplay. If you encounter that, use this:
aplay -c 2 -f S16_LE -r 48000 /dev/zero
This plays a MS-WAVE conform stereo 48kHz stream of absolute silence to the default output (in your case; pulseaudio)