It’s easy to tweak Planner. For example, the following two lines of
code delete all lines that contain \{\{private\}\} before publishing. Your
planner pages will be fine, but the published HTML won’t contain them.

(add-to-list 'planner-publishing-markup (lambda () (delete-matching-lines "\{\{private\}\}")))
(planner-update-wiki-project)

Fun, isn’t it?