MediaWiki links would directly work in Phabricator.
For example [[w:VisualEditor|]] would link to [http://en.wikipedia.org/wiki/VisualEditor VisualEditor]. The same for the other interwikilinks to sister projects (Wikibooks, Wikisource, and so on).
And the user could select use VisualEditor as default in preferences (in Wikimedia Phabricator).
Nowadays, it is very difficult to write some text to link to examples to Wikipedia, Meta or Wikimedia. This is not of interest or Wikimedia users.
https://secure.phabricator.com/book/phabricator/article/remarkup/
[[https://en.wikipedia.org/wiki/Main_Page|Wikipedia Main Page]] is already the syntax for external links - Wikipedia Main Page
There's little to no benefit from us writing our own syntax handling into this, as we'll have to maintain it, upstream won't want it.
If you want to past a link to Wikipedia, just paste the link. You don't need any fancy surrounding syntax. That is not difficult
Phabricator supports Markdown, which is less broken than MediaWiki's wiki markup (which has no proper definition anyway).
See https://www.mediawiki.org/wiki/Phabricator/Help#Formatting_and_markup
There are no plans to maintain a downstream patch to support wiki markup for years to come, sorry :)
Nitpick: Phabricator actually uses Remarkup, which behaves slightly different than Markdown does.