Hopefully, you have noticed, that a WorkItem is activated while one of its SmartParts respectively controls is entered?
It is independent to which of the WorkItem's collections (Item- or SmartPartCollection)  the control was added, it only has to be derived from System.Windows.Forms.Control.
This works due to the ControlActivationService of the CompositeUI.Winforms Library. If the control is entered the ControlActivationService activates the containing WorkItem. The SimpleWorkItemActivationService ensures that only one WorkItem is active.
Paulo Morgado shows us how to replace the SimpleWorkItemActivationService by an hierarchical ActivationService.