You should be able to apply an unlit material using a similar setup as the ARKitCameraMaterial (“AppleARKit Content” section when you switch on engine content) uses to any geometry you wont to have as a shadow catcher or occluder. Thats the way I’ve done it on previous AR projects using Vuforia and way back pointcloud.
I wish Oculus hadn’t bought pointcloud and shelved it, that tech was never perfect but it had stable SLAM on much lower specs than either ARKit or ARCore and that was years ago.