Understand your problem, but consider that there can be more than one selected document when more than one document pane are displayed. Also the active/selected content can be a document or an anchorable.
If you see how VS works you can have 4 types of "selected/active" contents: The ActiveContent is what has the keyboard focus (the property grid pane display an object from this pane), the LastFocusedDocument that is the tab inside a document pane (document or anchorable) that has last gained focus, the selected tab (anchorable or document) inside a document pane and the selected tab (anchorable) inside an anchorable pane. Sometimes these properties point to the same object.
If you want to bind directly to the ActiveContent you should consider using a converter that can filter out the anchorables, if you want the last activated document that use the property LastFocusedDocument using the same converter.
If you want the list of selected documents, you can just work with the model (pseudo-code):
dockManager.Layout.Descents().OfType<LayoutDocument>().Where(d => d.IsSelected);
Said that I'm convinced too that maybe something easier to get work with should be implemented and for that I'm absolutely open to discussion,
Ado