A bit late to the party but for any future readers, you can still use a sphere mask, but place it inside of a post-process material. Lerp between your unreal scene and your AR texture like so:
The important thing to remember is that you must update your “CameraTexture” Param2D every tick, so create a separate blueprint and feed it into your PostProcess material (as a dynamic material instance) once your AR session has begun:
I’ve tried using custom depth and ARKit in Unreal 5.0 but found some funny behaviour going on… it was many months ago and I don’t quite remember the issue, but IIRC it had to do with the UVs of my stencils getting improperly mapped as soon as I accessed the AR CameraImage texture. Maybe it’s fixed now 