I think this is a great idea. Not only useful for navigation, but also as overview of the current position in the document.
* Could be implemented as "Document Hierarchy pane" (similar to the "File pane").
* Should only show notes that are parents (have children).
* The section currently viewed could be highlighted, so you always know where you are in the current document.
* The current note (where the cursor sits) could be highlighted as well.
* The depth (number of levels shown) should be editable at the top of the pane (with + and - buttons), like [-] 2 [+].