This tab would contain (at least) the same controls as the current popup region editor. The main difference in thinkin would be that the editor would be able to affect all regions selected.
Zero regions selected
All values would be blank and all controls disabled. Also the text "no regions selected" should be visible.
One region selected
Controls enabled and all values accoring to the selected region. Equal to the current popup region editor.
More than one region selected
Controls for the values enabled and the visible values would be the values of the region selected (choose: last / first). The values which are differ between regions (at least one region has a different value) could be blanked / greyed out. Editing of such value would change this parameter to the selected value for all selected regions. After editing the value, the parameter will be the same for all regions, so the value could be shown afterwards. If the control is a "relative" control, the relative change would be mapped out for each region separately (in which case, the value would still be blank or greyed out).