Holy crap, I didn't know we could have mirrors in Godot VR. Why isn't this mainstream?
Viewing post in Mainspring jam comments
It requires a engine patch to do properly currently https://github.com/V-Sekai/godot_planar_reflection (https://github.com/godotengine/godot/compare/master...hiinaspace:godot:mirror-ov... is my patchset including a fix for the procedural skybox shader). But https://github.com/godotengine/godot/pull/116424 Bastiaan is working on this, so hopefully soon it won't.