Tryed this out on my OG Xbox One and it works Great!
If you want to get past the Blackscreen of Beetle PSX SW/HW you'd need to put the SCPH5500/01/etc. BIN Files into the Folder "System" of RetroArch. As far as i have read this is possible with a Tool called "DurangoFTP" but i havent tryed that out yet.
gamr13.github.io
(You can Download DurangoFTP through MS Edge on the Xbox afaik)
I went the direct Route of Starting my PSX ISO's with PCSX-Rearmed as it is able to Emulate BIOS.
After that i tryed Nintendo 64 Emulation and to my Surprise it Runs "Banjo Kazooie Beta" more Fluid than even my PC-Rig
Hydro Thunder also works great with most of the time running at 59.94 FPS (there are some slight slowdowns when a lot of NPC's are in the same Frame though)
I've tryed Sega Saturn Games too but these give me a "Unrecognized Media Error" same thing seems to happen to me with .CCD Files
And i Tryed PRDoom , and Doom to my Surprise also Runs at Full 60 FPS which is Great - with that i found out that Saving inside the "LocalState" Folder inside the Retail Xbox where RetroArch then is installed to is possible
Also dont enable FPS Display while in 3D Scenes as the Calculation of the FPS seems to Slow Down the Whole Console ...
The Biggest Problem i have had with RetroArch was that my Controller wasnt Recognized so i had to Set it up Manually with having the "Close Content" and the "Open Menu" Buttons set through a Button Combination (L1+SELECT = Close Content | L1+START = Restart Game | L1+R2 = Open Menu)
This then Enabled me to Setup the PCSX Rearmed to "Remove PS Anti-Colorbanding Dithering" which also yields a slight Plus of FPS.
EDIT : Oh and umm.. i would strongly suggest to keep your Console Offline. While looking through the Partitions i've noticed a Folder called "BlackBox" this seems to suggest that there is a Security Logger in Place. And i assume that when using DurangoFTP while Online and Logged into your MS Account the Blackbox thingy could throw a Check Flag at Microsofts Xbox Live Servers...
This is just an assumption though , as i know that the PS3 had similar Security Checks in correspondence to the Master Servers. Also Microsoft Store has it's own Partitions relating to Security Check Overlays (which makes sense on a Console). Plus , i was once tooling around with Forza Horizon 3 Debug , and as soon as you Modify just one Container everything breaks.
I'll leave mine on Offline and Logged Out just for Safety as i dont want to loose my Account just because im able to see the Inner Workings of a Retail Xbox One...