When higher priorities are dealt with, please consider adding a Note component that never publishes for adding notes directly on the page.
If you want to get fancy, add a drop zone to use for on-page components that you’ve configured but don’t want to publish at that time. It will have the advantage of notes with it on why it is not published yet.
Perhaps a switch to show/hide notes and similar Components that might be useful but really mess up the WYSIWYG view? Maybe there could be a class of Components that fit this category besides a Note Component?
I wonder if something along the lines of ‘invisible elements’ might work here. Note elements could simply be considered <!-- comments --> included in the page structure, and visible in the viewport as overlays (when set to visible), but not included when publishing.
I’m sure there might be other ideas for invisible elements (named anchors, etc), but the I think the idea of creating notes or comments within a page would add a lot of benefit.