The English Wikipedia developed a tool in JavaScript called "watchlist notices."
From http://en.wikipedia.org/wiki/Wikipedia:Software_notices#Watchlist_notice:
Shows to all users who use a watchlist. Appears above the [[horizontal rule]] on the watchlist. Controlled by [[MediaWiki:Watchlist-details]]. Dismissable using JavaScript found in [[MediaWiki:Common.js/watchlist.js]].
This is a rather hackish implementation (relying on an unrelated system message). This feature (including the cookie dismissability) should be implemented in a MediaWiki extension or in core. It'd be useful to have on other Wikimedia wikis.
Perhaps integrated with CentralNotice? Who knows.
Version: unspecified
Severity: enhancement