If it helps, I managed to extract ac3 from the Digiface USB spdif input but not in a way that helped me.
It took a while but eventually I learned that the spdif signal is not pure ac3 but ac3 wrapped as SMPTE ST 337. VLC does not recognise this but ffmpeg and MediaInfo do.

So if I first capture the first two Digiface channels into a file using:
`ffmpeg -f avfoundation -i :2 -filter_complex "[0:a]pan=2C|c0=c0|c1=c1" -y capture.wav`

and pipe this file into the VLC audio capture device via Blackhole2ch or any other audio device:
`ffmpeg -i capture.wav -f spdif -f audiotoolbox -audio_device_index 2 -`

it works since ffmpeg will extract the ac3 and if necessary, decode appropriately.

However, if I try to use VLC as the capture device directly:
`ffmpeg -f avfoundation -i :2 -filter_complex "[0:a]pan=2C|c0=c0|c1=c1" -f spdif -f audiotoolbox -audio_device_index 2 -`

ffmpeg does not extract the ac3 but passes on the raw signal as PCM!

I tried bypassing the problem by using a pipe, such as:
`ffmpeg -f avfoundation -i :2 -filter_complex "[0:a]pan=2C|c0=c0|c1=c1" -f wav - | ffmpeg -f spdif -i - -f audiotoolbox -audio_device_index 2 -`

Here the second ffmpeg recognised the format and this worked until decoding errors poisoned the stream.

Nothing I tried solved this problem, so unless I get another bright idea, I guess I am giving up on this for now.