I stole it from the EventLogging extension - you can see it still on https://meta.wikimedia.org/wiki/Schema:AccountCreation. It's a couple of files / functions that can be stolen from EventLogging and reused.

Change 123159 had a related patch set uploaded by Brian Wolff:
Mark GWToolset namespace as being json for code editor.

https://gerrit.wikimedia.org/r/123159

(In reply to Gerrit Notification Bot from comment #2)

Change 123159 had a related patch set uploaded by Brian Wolff:
Mark GWToolset namespace as being json for code editor.

https://gerrit.wikimedia.org/r/123159

This is just for half the bug. I didn't do anything related to showing json on page view, just the bit about telling [[mw:Extension:CodeEditor]] that the GWToolset is json, so the edit box gets syntax highlighting.

Change 123159 merged by jenkins-bot:
Mark GWToolset namespace as being json for code editor.

https://gerrit.wikimedia.org/r/123159

Resetting to new for the event-logging style json visualization.

The code editor thing should start appearing on april 8.

https://gerrit.wikimedia.org/r/#/c/134167/

This should be pretty easy to implement now. Someone would need to set the default content model to 'json' for the GWToolset namespace, and then write a maintenance script to covert the existing pages into JSON content ones.