There is not yet any community consensus for the deployment of this extension, but I thought I would get the ball rolling.
The [[mw:Extension:PageNotice]] extension allows the display of an arbitrary notice, similar to SiteNotice, but at the top of a page's content area, below the tagline. The notices can be set up on a per-namespace bases (or per-page basis, if not disabled in LocalSettings).
There are currently three Gerrit changes awaiting review, that bring the code up to the standards expected of a modern MediaWiki extension (Gerrit change 105141), and add a couple of minor features (Gerrit change 105142, Gerrit change 105143).
The initial motivation for this extension is to allow for a notice to be displayed at the top of every page in the Draft: namespace on enwiki.
Other use-cases include:
* a standard header on all article talk pages,
* a standard notice on Wiktionary Reconstruction: namespace pages (described in comments below, with community support),
* a standard CC-O notice on Mediawiki-wiki Help: namespace pages,
Here is "Greg's checklist" for this extension:
== TODO/Check list ==
Extension page on mediawiki.org: YES
Bugzilla component: YES
Extension in Gerrit: YES
Design Review: NO, but since it is so simple, I doubt it really needs one
Architecture/Performance Review: NO
Security Review: YES
Screencast (if applicable): N/A
Community support: NOT YET