function para redirecionar templates e singles de um plugin para /templates

plugin_template_redirect.php

define( 'PLUGIN_PATH', plugin_dir_path(__FILE__));
add_action('template_redirect', 'load_template_pluginname');

function load_template_pluginname(){
    global $post;

    if(get_post_meta($post->ID, 'pluginname_page', true)){

        if(locate_template('template-plugin-name.php', false) == ""){

            require_once PLUGIN_PATH . '/templates/template-plugin-name.php';
            die();
        }
    }

    if(is_singular('plugin-name')){
        if(locate_template('single-plugin-name.php', false) == ""){
            require_once PLUGIN_PATH . '/templates/single-plugin-name.php';
            die();
        }
    }
}