allow customizer to use the mapped domain when WordPress MU Domain Mapping is set to "Redirect administration pages to site's original domain"

dm_redirect_admin_customizer.php

function dm_redirect_admin() {// line 607 on WordPress MU Domain Mapping 0.5.5.1
	// don't redirect admin ajax calls
	if ( strpos( $_SERVER['REQUEST_URI'], 'wp-admin/admin-ajax.php' ) !== false )
		return;
	global $wp_customize; // gets $wp_customize global
	if ( get_site_option( 'dm_redirect_admin' ) && !isset($wp_customize) ) { // redirects to original url only if $wp_customize is not set
		// redirect mapped domain admin page to original url
		$url = get_original_url( 'siteurl' );
		if ( false === strpos( $url, $_SERVER[ 'HTTP_HOST' ] ) ) {
			wp_redirect( untrailingslashit( $url ) . $_SERVER[ 'REQUEST_URI' ] );
			exit;
		}
	} else {
		global $current_blog;
		// redirect original url to primary domain wp-admin/ - remote login is disabled!
		$url = domain_mapping_siteurl( false );
		$request_uri = str_replace( $current_blog->path, '/', $_SERVER[ 'REQUEST_URI' ] );
		if ( false === strpos( $url, $_SERVER[ 'HTTP_HOST' ] ) ) {
			wp_redirect( str_replace( '//wp-admin', '/wp-admin', trailingslashit( $url ) . $request_uri ) );
			exit;
		}
	}
}

dm_redirect_admin_original.php

function dm_redirect_admin() {// line 607 on WordPress MU Domain Mapping 0.5.5.1
	// don't redirect admin ajax calls
	if ( strpos( $_SERVER['REQUEST_URI'], 'wp-admin/admin-ajax.php' ) !== false )
		return;

	if ( get_site_option( 'dm_redirect_admin' ) ) {
		// redirect mapped domain admin page to original url
		$url = get_original_url( 'siteurl' );
		if ( false === strpos( $url, $_SERVER[ 'HTTP_HOST' ] ) ) {
			wp_redirect( untrailingslashit( $url ) . $_SERVER[ 'REQUEST_URI' ] );
			exit;
		}
	} else {
		global $current_blog;
		// redirect original url to primary domain wp-admin/ - remote login is disabled!
		$url = domain_mapping_siteurl( false );
		$request_uri = str_replace( $current_blog->path, '/', $_SERVER[ 'REQUEST_URI' ] );
		if ( false === strpos( $url, $_SERVER[ 'HTTP_HOST' ] ) ) {
			wp_redirect( str_replace( '//wp-admin', '/wp-admin', trailingslashit( $url ) . $request_uri ) );
			exit;
		}
	}
}