template_personalizado.php

<?php 
function rewrite_form_envio_cupula_dos_povos($wp_rewrite) {
    $new_rules = array(
        "cupula-dos-povos/form-envio" => 'index.php?meu_template=form_envio_cupula_dos_povos',
    );
    $wp_rewrite->rules = $new_rules + $wp_rewrite->rules;
}

add_action('generate_rewrite_rules', 'rewrite_form_envio_cupula_dos_povos');

function registrar_query_vars($public_query_vars) {
    $public_query_vars[] = "meu_template";
    return $public_query_vars;
}

add_filter('query_vars', 'registrar_query_vars');

function template_redirect_intercept(){
    global $wp_query;

    // verifica a variável
    if ( $wp_query->get('meu_template') == 'form_envio_cupula_dos_povos' ) {

        // se tiver o valor que queremos, usa o nosso template
        if (file_exists( TEMPLATEPATH . '/form_envio_cupula_dos_povos.php' )) {
            include( TEMPLATEPATH . '/form_envio_cupula_dos_povos.php' );
            exit;
        }
    }
}

add_action('template_redirect', 'template_redirect_intercept');