If security by obscurity is not sufficient you may need to have your notes in encrypted tiddlers. Once MWS multiwiki server is mature you will be able to publish wikis with suites of tiddlers not readable by some users. However I recommend you design your wiki so the notes are not part of the content but in a related tiddler. This gives you the necessary control over the content which you want to make conditional.
Over hear Tips and commentary - Notes on tiddlers stored in a data tiddler I wrote
Perhaps we could extend this to use a simple form of encryption to store the notes, and use a bookmarklet in your browser to install the key in a tiddler, but not save this tiddler with the wiki. So you just click a bookmarklet to read and edit the notes but visitors will not see them even if they open the note database.