VSIX: How to dock a toolwindow in the MainWindow

Writing a Visual studio extension, I'd like to write tabs in the main window that are persisted and not backed by files (documents) but by data that gets loaded form a web service. The most promising avenue of a solutions seems to be a tool window with multiple views, as it promises to handle persistence across VS sessions as well.

Now I found the Microsoft.VisualStudio.Shell.Interop.ToolWindowGuids class to set the Window property for a tab style docking by default. However this lists only typical tool window areas. Where can I find the GUID or a reference for the DTE.MainWindow which seems to be the right place to dock the multiple views of the tool window?

Thanks

Kaj

P.S.: Extension is a package