Wow. I’d love to see if this is possible. I had to read your post twice to understand it.
You might want to try checking if the UHD HFR release is right or left eye only. If it is, you might possibly downscale the 4k source, and then take the missing eye side from the 3D BD, and then use an AI framerate increase on it.
Then you can skip depth maps, etc. Sure the extrapolate side will have lower quality, but when viewed in 3D the difference should smooth out.
I’d love to see this… assuming I have hardware capable of playing it.
Of course my preference would be the Hobbit films, since GM isn’t supposed to be very good and I skipped seeing it.
Edit: As far as the Hobbit goes, I’m fairly certain there is no such thing as 1080p48 on anything, 2D or 3D.
I always figured a HFR 2D release could be created (if you had a HFR source) by doing a double telecine to 60 fps and then decombing it. I suppose the same idea might hold for a 3D version.
Edit 2: Apparently my 8 year old Sony with Panasonic active 3d glasses handles 1080p60 3D just fine. I had my doubts.
I’m assuming you’ll keep the frame rate at the 59.94 that the UHD BD uses so you don’t have to re-encode the audio.
For anyone curious, here is a test I created. The source is the ‘Big Buck Bunny’ short (which has a Creative Commons license, so should be okay to post here). I split it, resized, restacked, re-encoded and slowed it to 1080p59.94 Half Side By Side 3D with Dolby 5.1 audio.
https://www.mediafire.com/file/e3xx8s70lc1terx/Big_Buck_Bunny_Half-SBS_3D_1080p59.94.mkv/file