For vrchat map makers out there, I figured out how cameras like vrclens and virtuallens take over the vrchat camera without affecting the VR view, so you can make your own gimmicks in your maps. It's a pretty simple trick: since VR headsets render in nonstandard resolutions, detect if the output of the shader is 16:9, and if so, stretch a quad (with the camera render texture) over the entire screen in clip space, else (it's the headset), shrink to zero so nothing is rendered.

The second trick is to make the quad with the material on it have its four vertices in the corner of a cube, so that the mesh's axis aligned bounding box is a full cube. That way, when it's scaled up huge, unity will never frustrum cull it, wherever the player's vrchat camera is in the world.

unitypackage with demo scene: https://files.catbox.moe/8njljs.unitypackage