I changed the approach to solve the problem and decided to use jQuery. I proceeded to create
<div id="checkbox_emisores">
Then en _form.php would look like this:
<div id="checkbox_emisores">
<?php echo $form['receptores_list']->renderError() ?>
<?php echo $form['receptores_list'] ?>
</div>
Here the function to replace tags for checkboxes generated by Symfony 1.4 by the tag you need Bootstrap. I adapted the function as shown here:
jQuery : A simple tag replacement
$(document).ready(function(){
$.ajaxSetup ({
cache: false
});
// First delete the label generated by symfony
$("#checkbox_emisores").find('label').each(function(index) {
var htmlStr = $(this).html();
$(this).replaceWith(htmlStr);
});
//Then replace the symfony <li> with <label generated class='checkbox'> as needed in Bootstrap
$("#checkbox_emisores").find('li').each(function(index) {
var htmlStr = $(this).html();
$(this).replaceWith("<label class='checkbox'>"+htmlStr+"</label>");
});
});