I guess it makes more sense to change the rendering instead of changing ~12800 database entries[1]. However...

• if we added a new file like src/applications/maniphest/remarkup/ManiphestMWCodeReviewRemarkupRule.php
  • with getObjectNamePrefix() returning 'r'
  • and getObjectIDPattern() returning '1?\d{0,5}'
  • and getObjectHref() returning something like 'https://static-codereview.wikimedia.org/MediaWiki/' . $uri . 'html',
• and we ran arc liberate and added that file to the return array of src/applications/maniphest/application/PhabricatorManiphestApplication.php::getRemarkupRules() so it'll be applied,
• we'd still have src/infrastructure/markup/rule/PhabricatorObjectRemarkupRule.php defining abstract protected function loadObjects(array $ids), thus our new file must also implement loadObjects. But what to load?
• In any case, if (empty($objects[$spec['id']])) will be true in PhabricatorObjectRemarkupRule::didMarkupText() as any object (e.g. r12345) does not exist in Phabricator itself (while objects like T12345 exist), and that code says For objects that are invalid or which the user can't see, just render the original text.

Looks like auto-linking to external stuff isn't that trivial to implement.
But was an interesting afternoon investigation and might also be useful for T687.

[1] SELECT COUNT(mtc.content) FROM phabricator_maniphest.maniphest_transaction_comment mtc INNER JOIN phabricator_maniphest.maniphest_transaction ta INNER JOIN phabricator_maniphest.maniphest_task t WHERE ta.objectPHID = t.phid AND mtc.phid = ta.commentPHID AND t.id < 75682 AND mtc.content REGEXP " r[0-9][0-9]+";

Doesn't your previous comment only mean that you would have to subclass PhutilRemarkupRule directly rather than PhabricatorObjectRemarkupRule? I note that Wikimedia Phabricator already has custom code to allow embedding videos from Wikimedia Commons, so it clearly is possible to add markup rules for external stuff. (https://phabricator.wikimedia.org/source/phabricator/browse/wmf%252Fstable/src/infrastructure/markup/rule/WikimediaCommonsRemarkupRule.php)