Editor: Drawing outside the own window

Is there a way to draw outside an own EditorWindow's position rectangle?

I looked into the internal class "GUIClip", and it seems it handles the clipping of rectangle areas, but beside calling GUIClip.Pop() repeadately (which leaves the GUI stack corrupt) I can't figure out how I could, e.g. temporarely disable the clipping.

Or does anyone know of a callback where I can draw using GUI functions that is "after" all repainting?

To provide a bit of context: I am trying to inject another drop-down control next to the "Layers" drop down in the Toolbar view on top of the editor. There is so much unused space and there would be a very nice place.... ;).