Using sitenotice for any messages other than the actual sitenotice is a really bad idea: when the user hides a sitenotice, any other messages, like the Collection toolbar, are hidden and not showable without undoing the sitenotice hide. I’m not sure if it even has to be above the page title, it might be moved to #contentSub (just below the tagline), or even to the bottom of the page or to the toolbar (where there is already a disable button).
Description
Description
Event Timeline
Comment Actions
The Collection extension is needed to be modified, but I’m not familiar with the code so I can’t say it more exactly.
Comment Actions
This is about the following interface, which the Collection extension injects at the top of every article if the current user has an unfinished "book" collection that they started on Special:Book.
The code for that is in mediawiki-extensions-Collections.git:/includes/CollectionHooks.php#siteNoticeAfter.