it would need an additional tab because it is a very different approach
A good point, and as you say it should be independent of any choosing of autosave settings, but as @kugel- noted, the general context is suitable for that plugin (and its way easier to extend an existing plugin than to make a new one).
To be clear, for contents to be saved between invocations of Geany you are going to have to save them somewhere, and then they will have a unique name, even something useful like `scratch-1` scratch-2` etc. so if the plugin does that it should avoid the save query.
To be clear, the editing notebook is for _files_ it doesn't handle anything else, and changing that is a big change.
Your other option is to add tabs to the message window, like extra scribble tabs, but that would be a whole new plugin.