WordPress 302 Redirect to Holding Page

I would place the redirect inside WordPress. It lends you more control, and you don’t need to worry about WP conflicting with your own .htaccess rewrites anymore.

function wpse_147377_redirect_to_holding() {
    if ( ! is_page( 'holding' ) ) {
         wp_redirect( site_url( 'holding' ) );
         exit;
    }
}

add_action( 'template_redirect', 'wpse_147377_redirect_to_holding' );

Place it in a “Must Use” plugin (wp-content/mu-plugins/my-plugin.php), a regular plugin, or your theme’s functions.php.