As I mentioned over on the HQPlayer integration thread, getting this working was remarkably simple.
I already had Volumio running on my Raspberry Pi 2 + Hifiberry Digi+ card and an Edimax EW-7811UTC wifi adapter for high-speed 802.11ac connectivity (which I needed for playback of higher-resolution DSD & 192kHZ PCM files). Worked fine, but Volumio was so kludgy,
I did a fair amount of reading the past few days, and remembered reading that the HQPlayer NAA stuff could work alongside MPD. So on a whim I tried a variant of something I saw on a post by Miska on computeraudiophile.com.
I ssh’ed into my RPI2 and tried:
apt-get install libasound2 alsa-utils
wget http://www2.signalyst.com/bins/naa/debian_wheezy/raspberry_pi/networkaudiod_2.0.5-20_armhf.deb
dpkg -i networkaudiod_2.0.5-20_armhf.deb networkaudiod_2.0.5-20_armhf.deb
I went back into HQPlayer Desktop on my iMac, changed the Backend to NetworkAudioAdapter and much to my surprise, under Device there was snd_pi_hifiberry_digi! I changed the SDM Pack to DoP and was in business.
To simplify things I first played back a local file from HQPlayer, but once it worked, I switched over to Roon—actually RoonServer + HQPlayer running on my iMac, controlled by the Roon iPad app.