There are docs for a Lumin Runtime ExclusiveRender object, accessed from LandscapeApp::startExclusiveModeGL. The docs hint that this is a way that landscape apps can enter immersive mode without having to do inter-process communication, which is exactly what we want for WebXR.

According to https://forum.magicleap.com/hc/en-us/community/posts/360048535712-ExclusiveRender-examples, this functionality is disabled for security. Indeed, trying to use it just gets a null pointer back from startExclusiveModeGL.