Custom field display

There is no option for that.
It's possible to do it, if you have some PHP skills, by editing the file show of the view product and storing the HTML of the specifications somewhere without displaying them and then creating a module which would just display the HTML. Then, with Advanced Module Manager, you can set that module to only display if there is something in it.

Without AMM, you can still do that but you have to edit your template to not display the load position of the module when not on a product page.

I didn't look at the mercado files so I can't say.

Yes. That's what I wrote earlier:

It's possible to do it, if you have some PHP skills, by editing the file show of the view product and storing the HTML of the specifications somewhere without displaying them and then creating a module which would just display the HTML.

You can't just copy parts of the show file in the module.
The show file needs the data from the view.html.php file in order to display it. In your module, you won't have that data available.
The easiest is to put around the tags below around the code you want in your module:
<?php ob_start(); ?>

and

<?php $_REQUEST = ob_get_clean(); ?>


Then, in your module you can just have the code: <?php echo $_REQUEST; ?>

You have one show file per template on your website. If you edit the show file for the beez template, the modification will go in the bee template, if you edit the show file of your main template, it will put it in the main template.
That allows you to have different modifications for different templates on your website (you can use several templates on a joomla website).