I am perfectly able to create and use a custom observable for a mesh, but not for a scene. Is this a bug or a feature?
Could you provide a repro in the PG (even if it fails), as I’m not sure to fully understand your request?
Have a look here how a custom observable was added to Scene for physics. It’s patched to scene prototype (there is code lower down also for AbstractMesh). Is this how you are doing it?