After painful investigation and comparison with some examples provided by Oculus SDK, I could make it work using 2 tricks:
-using an extra layer of grouping above the group of objects and adding the rigidbody at that level
- and in below group, having:
-- "interactable group view"
-- and usual Grab interactable, Hand Grab Interactable, Grabbable (and in my case a One GRab Rotate Transformer")
- and having Box Colliders attached to the various objects of the group.
I don't know if that's the most elegant way to handle, but it works..
