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
.