Ach. To be clear, I use PowerDVD because it will bitstream, not so much for any viewing ability, though it does have two modes, for pc or tv.
If it's a NAS setup, couldn't you run a software player from it? I confess I don't know all that much about the commercial NAS units.
I just have a big ass pc with a lot of drives, and a SAS/SATA card to run them all and to create RAID arrays with the MaxView Storage Manger (works with my Adaptec SAS/SATA card).
Otherwise unless someone has another solution, I think a pc will have to be in the chain somewhere, or some device to serve the music to the AVR, unfortunately. I like the functionality of the Asus mini-pc's but a used laptop with HDMI could fill the gap.
Anyway like you say it's a shame network playback isn't incorporated into AVR's. I mean they often come with Tidal, Bluetooth, Chromecast and all that, and a plethora of stereo inputs, none of which I personally find useful. I even have a phone app for my Onkyo though I seldom use it.