Use groups to create your global overlay:
Design one of your scenes and also create all the sources you intend to be your global overlay within that scene. Multi-select all sources that define your global overlay, right-click them->Group selected items. Give the group a meaningful name.
Now change to a different scene where you want your overlay to appear as well. Click Add->Group->Add Existing and add your global overlay group. Whenever you change something within that group, changes will appear to all scenes as well where this group is included.