Cannot retrieve a custom RSS field from posts

The goal is to display a featured thumbnail micro thumb as a custom field in RSS.

This is what I’ve done:

    //theme functions.php
function rss_post_thumbnail($content) {
    global $post;
    add_post_meta($post->ID, 'little_thumb', get_the_post_thumbnail($post->ID,  'thumbnail'), 1);
    return $content;
}
add_filter('the_excerpt_rss', 'rss_post_thumbnail');
add_filter('the_content_feed', 'rss_post_thumbnail');

    //feed rss2.php
<item>
    <little_thumb><?php echo get_post_meta(get_the_ID(), 'little_thumb', true); ?></little_thumb>
</item>

But, I can’t seem to retrieve this value with the parses. If I open RSS source code though, I can see this thumb right there.

Any ideas?

I’m using a really lightweight lastRSS.php class, if it matters.