'Layouts' have a Toolbar in the .mns file, search for "TB_LAYOUTS". You could modify the ToolBar section. I cannot find the 'right-click' from layout tab section in the .mns file.
check in the acad.mns for the following: ***POP505
**OBJECT_VIEWPORT
[Context menu for Viewport Object]
that is were the additional items appear in the right click menu.