While I wouldn't mind having this feature, it isn't going to help in all cases as not all DACs accept DSD over SPDIF, many DACs only support DSD over USB, and relatively fewer DACs support DSD over SPDIF.
Since the WiiM Pro does not have a USB output, a better solution might be to offer transcoding to hi-res 24-bit PCM on the fly, as many open-source distros such as Volumio or Moode do.
Both of the above would be the ideal addition of course, and I'm guessing it might only be possible on the Pro and not the Mini.