Can you search by "ULTRA HIGH DEF" for "24/192" ?
Well,
yes, but what do you get in return?
I tried searching on "ultra high def", and it returned a lot of what appear to be playlists in "HD" (so, really returning things with that in the title description).
The selections within those playlists DO appear to be all "UHD" (by their definition), including 24b44.1 tracks...but I have not sampled very many yet.
See screen shot, attached.
So it looks like the full truthful answer is "sort of", but probably
not what you were hoping for, and (slightly) better than a poke in the eye with a sharp stick.
"24/192" returns only tracks/playlists with that phrase in the title.
I found most of the CDs I had in my wish list are there (so far all at 44.1, haven't tried searching on SACDs yet).
Still, I like it, and have even pre-ordered the (just released) new Roku Ultra streamer - hdmi, of course, and who knows what/if Amazon will stream multi-channel discs.
I did load the Amazon Music app on my old 'plain' Roku, and it taps into the same "HD" UI as on my desktop, but haven't played with that much yet, so don't know if it even 'transmits' "UHD" via that device.