Please, please, extend the UIDOC.selection with the elementId's of selected elements in linked files.
When you now select elements in a project and ask the API to give a list of selected elements, it will only return the elements in the main file and leaves out all selected elements in a linked file.
When this is possible, this idea is solved aswell. So what you say Autodesk? One addition to solve two questions 
highlight-element-selection-in-linked-files