SteamOS on Quest Pro.
Looks like SteamOS is "running" on the Quest Pro.
The login screen initially wasn't working because the hardware video decoder was disabled, and that turned out to be caused by missing firmware. Once that was fixed, login started working.
There are still a few big things left to solve: Wi-Fi, Bluetooth, and, most importantly, tracking.
The system is currently very unstable, and tracking doesn't work yet, so it's being controlled manually with a Python script. However, rendering is working, including accurate lens distortion, which is a pretty big milestone.
As for getting past the Quest's bootloader: the project uses a custom kexec implementation.
Next step is making everything more stable and getting the hardware features fully working.
Credits: original updates by @marquina_osu on X; compiled and edited by me.