If you use this extension to create redirects to a category page, you categorize it with that category instead of redirecting.

$crRedirectTitle = $wgRequest->getText( 'crRedirectTitle' );
...
$wpTextbox1 = "#REDIRECT [[$crRedirectTitle]]\r\n";

Seems like we need to check for "Category:" in front, and prepend a colon. Or maybe, just maybe, use a Title object. (And doesn't edit page trim trailing whitespace anyway?)

Could be accomplished as part of T28898.