I have an Activity called CustomActivity with a Property Source to hold another activity. For getting the reference to the ActivityDesigner of teh Activity referenced by teh Source porperty, I can use the following code
(cusomActivityModelItem.Properties[“Source”].Value).View
cusomActivityModelItem => refers to the ModelItem of the CustomActivity
cusomActivityModelItem.Properties[“Source”].Value => refers to the Activity assigned to the Source property.
(cusomActivityModelItem.Properties[“Source”].Value).View => returns the ActivityDesigner corresponding to the referenced Activity