I try to create an area onto a CDialog, where I can put some CDockablePanes in. Those shall be perfectly dockable to a fixed dialog content.
The Codejock Dialog Panes Sa
Another simple method is to set your docking mode to DT_IMMEDIATE if you are in a Dlg implementing Docking Frames. Call CDockingManager::SetDockingMode(DT_IMMEDIATE);
in you CFrameWndEx object's OnCreate (or someplace appropriate).
Okay, I finally got it.
Instead of letting the MFC create the dummywnd, I created it by myself. So the MFC skips the creation and the call to GetTopLevelFrame().
Now the problem was, that the dummywnd member variable was protected and had no public set method. So I inherited a class and built myself a public set method.