через хук-фильтр posts_search
например:
function q418923_chg_search( $search, &$wp_query ){
    global $wpdb;
    if ( empty( $search ) )
        return $search; // skip processing - no search term in query
    $q = $wp_query->query_vars;
    $n = ! empty( $q['exact'] ) ? '' : '%';
    $search =
    $searchand = '';
    foreach ( (array) $q['search_terms'] as $term ) {
        $term = esc_sql( like_escape( $term ) );
        $search .= "{$searchand}( $wpdb->posts.post_title LIKE '{$n}{$term}{$n}' )";
        $searchand = ' AND ';
    }
    if ( ! empty( $search ) ) {
        $search = " AND ({$search}) ";
        if ( ! is_user_logged_in() )
            $search .= " AND ($wpdb->posts.post_password = '') ";
    }
	return $search;
}
add_filter( 'posts_search', 'q418923_chg_search', 500, 2 );
Мб можно сделать лучше. Но надо рыть в эту сторону.