They already exist some ways to put content above a page, like:
- the very useful MediaWiki:Scribunto-doc-page-name that allow the transclusion of the Lua module documentation from a subpage (example · reference)
Having said that, it seems that no system message is displayed above a JSON page and as far as I know it does not exist an MediaWiki:Article-nonwiki-header-page-name or whatever.
Like the behaviour of every Scribunto module, for articles with the JSON content model we could automatically try to display a subpage and transclude it on the top.
So we can create a new system message called MediaWiki:Article-nonwiki-header-page-name or whatsoever to keep the value of that subpage like $1/header.
I don't think that anyone has never created a /doc or /header subpage under a JSON page so I think it will not create any strange impact. Anyway, thanks to the system message, it could be personalized per-wiki to overcome every need.