Geo.hooks.php

<?php
/**
 * Hooks for Geo extension.
 *
 * @file
 *
 * @ingroup Extensions
 * @ingroup EventLogging
 *
 */

class GeoHooks {
	/**
	 * CustomEditor hook handler
	 * @see https://www.mediawiki.org/wiki/Manual:Hooks/CustomEditor
	 *
	 * @param Article $article
	 * @param User $user
	 * @return bool
	 */
	public static function onCustomEditor( $article, $user ) {
		if( $article->getTitle()->getNamespace() === NS_MAP ) {
			$output = $article->getContext()->getOutput();
			$output->addHtml( 'DO THE MAP EDIT INTERFACE' );
			return false;
		}
		
		return true;
	}

	public static function onAfterFinalPageOutput( $out ) {
		$out->clearHtml();
		$out->addHtml( 'yo' );
		return true;
	}
}

Geo.php

<?php
/*
 * This file is part of the MediaWiki extension Geo
 *
 * VectorBeta is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 2 of the License, or
 * (at your option) any later version.
 *
 * VectorBeta is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with Geo.  If not, see <http://www.gnu.org/licenses/>.
 *
 * @file
 * @ingroup extensions
 */

define( 'NS_MAP', 42 );
define( 'NS_MAP_TALK', 43 );
$wgExtraNamespaces[NS_MAP] = "Map";
$wgExtraNamespaces[NS_MAP_TALK] = "Map_talk";

// autoload extension classes
$autoloadClasses = array (
	'GeoHooks' => 'Geo.hooks.php',
);

foreach ( $autoloadClasses as $className => $classFilename ) {
	$wgAutoloadClasses[$className] = __DIR__ . "/$classFilename";
}

$wgHooks['CustomEditor'][] = 'GeoHooks::onCustomEditor';
$wgHooks['AfterFinalPageOutput'][]  = 'GeoHooks::onAfterFinalPageOutput';