I suppose this could just be a hook in wmf config too?

Probably, but writing a trivial extension sounds better from the maintainability standpoint to me. Easier to find the code if it's in mediawiki/extensions/SandboxLink rather than operations/whatever/wikimedia-config-blah.

Yeah, extension sounds good.

I'm not sure Bartosz was volunteering. :-)

The extension's requirements and architecture are still a bit murky to me. Presumably you'd want the ability to specify a link text message, a link target message, and an insertion point?

I am indeed volunteering. I was meaning to do this already, but got distracted.

The extension is meant to provide the exact same functionality as [[w:MediaWiki:Gadget-mySandbox.js]], with the exact same set of customizable (via system messages) options (see the 'conf' entry in the gadget's code).

Change 157297 had a related patch set uploaded by Bartosz Dziewoński:
Add SkinPreloadExistence hook

https://gerrit.wikimedia.org/r/157297

Change 157296 had a related patch set uploaded by Bartosz Dziewoński:
Initial extension code

https://gerrit.wikimedia.org/r/157296

Change 157297 merged by jenkins-bot:
Add SkinPreloadExistence hook

https://gerrit.wikimedia.org/r/157297

Change 157296 merged by Legoktm:
Initial extension code

https://gerrit.wikimedia.org/r/157296