I would love to see such capability in roon, but the issue is definitely not what HDMI can handle but what Dolby would permit a software solution like roon to decode and encode internally. If roon server would handle an Atmos stream even in a very basic way (i.e. for sending it via RAAT or doing level normalization or applying DSP) it would have to do significant Dolby-proprietary procedures i.e. possessing a significant portion of the ´secrets´ of Dolby’s algorithm. Unlikely that Dolby would allow that even if licensing fees would be a solvable issue.
Dolby Digital Plus in Tidal, Apple Music or Amazon Music is a different thing. It is a lossy format which is handed over by the aforementioned services and all the hardware in between without ever being encoded or decoded except from a Dolby Atmos licensed hardware renderer (such as an AVR). Same applies to ´true´ Dolby Atmos streams being in a closed container file such as MKV or MP4.
I doubt that roon would want the possibility to just hand over such stream untouched, particularly as Tidal seemingly is the only common source for streaming Atmos which roon could process (and the one with poorest bitrate according to my knowledge). File formats carrying a true Atmos stream are rare to find and ripping blu-rays is not really an easy nor a legally permitted thing.
Although I would love to see these options, I guess we are far from that.
What might be easier is an immersive format with less licensing restrictions. Auro3D in a WAV container might be easier to handle. 4.0.2 (commonly known as “2+2+2”) in a 5.1 FLAC container already works flawlessly with roon.
DR says nothing about audible sound quality or perceived dynamics, particularly if you compare formats with different number of channels or different level normalization.
This also does not say anything as you seemingly have not gone through the procedure of level normalization for each source and you have no knowledge about mastering steps taken for the different versions.
Most of them had pretty bad experience with introducing SACD, DVD and blu-ray some 20 or 25 years ago so do not blame them. The market for multichannel high end audio (home cinema excluded) is pretty thing. Unfortunately, I might want to add, as it indeed can sound superior compared to two channel stereo and in many many cases it does. Particularly with classical music, jazz and rock.
You mean the ´Hell freezes over´ version?
Would also recommend to listen to a proper multichannel or Atmos or Auro3D setup. If it is proper and the mix is well done, it will be outperforming stereo by far. Particularly if there is ambience information i.e. reverberation patterns of the recording room on the rear and immersive channel which is the case with all classical recordings and lots of jazz and (live) rock. It simply sounds wider, more natural, effortless and more like a concert hall, balancing direct and indirect parts of the sound field much better.