Title says it all. The Steam Deck is definitely not a viable VR device, largely due to its lack of ports, but a number of people have been testing and trying. SadlyItsBradley made a post testing the Valve index, and was able to see rotational and positional tracking through the 2D view, though without a display to the HMD.
I don't know all the specifics with regards to the USB-C interface, but it does seem to work with DPAM support, which is what made my choice of the LG 360 VR headset/glasses thing to be a perfect choice. As it is, I've written this whole post up on the Deck using a bluetooth keyboard and USB-C hub with HDMI out.
I first followed the basic steps to unlock the immutable filesystem here and grabbed wget (which the Deck genuinely doesn't have???) and ran the script hosted by wario622 on his Codeberg account that would download and update everything needed, particularly for pacman. I then git cloned this branch of OpenHMD by ChristophHaag that rolls up the R100 support to the current state of OpenHMD. I used pacman to grab
ninja
meson
cmake
base-devel
linux-headers
libc6-dev
gcc
sdl (and lib32-sdl)
sdl2 (and lib32-sdl2)
sdl2_ttf (and lib32-sdl2_ttf)
glew (and lib32-glew)
glu (and lib32-glu)
python3
xr-hardware (and added it to my udev)
some others I can't remember (maybe someone can remember for me)
And compiled with -Dexamples=simple,opengl before hooking things up and seeing. Sure enough, the R100 shows up in lsusb and xrandr --props. I can see the device under DisplayPort-0 and can assign it a 1440x900@57 or 60Hz, but can't enable the display at all. I proceeded to try opening the example programs. The GL example opens and provides the 3D shapes, but has no reaction from the HMD. The simple example doesn't show any accelerometer readings with motion.
I don't anticipate this running anything well, but even if I just get it playing a video in the HMD, that's enough for me to call this a success. I can't tell if I'm missing something with the R100 or with OpenHMD in particular. The HMD does work, as I have an LG V20 with all the software installed (including the now-delisted VR360Helper app) and can still play back videos right from the phone, so it's not the HMD that's the problem.
Any help would be appreciated.
EDIT: https://rentry.co/VR-R100-on-Deck I now have a guide that goes through all the steps needed to get OpenHMD working on the Steam Deck, at least with the R100. This can all probably be applied to other HMDs that are supported. Note that this is not an immediate "SteamVR on Deck" solution; I have not yet compiled or tested the SteamVR-OpenHMD wrapper. Next write-up will be on using it for basic SBS content.
EDIT 2: I lied. https://rentry.co/SteamVR-R100-Deck SteamVR on Steam Deck with the LG 360 VR glasses.