You can do this by opening the "Camera settings" dialog (either via the Display menu or via the small camera icon on the left toolbar).
Inside the lower frame, under the word "pivot" there's a 'point picking' icon which will let you pick a point in the current 3D view. This point will be used as the global rotation center (alternatively, the "magnifying glass" icon sets directly the selected entity center as pivot).
Using per-cloud rotation center is interesting, however it will imply some non trivial change in the current architecture. I'll add this to the TODO list.
Daniel, CloudCompare admin