Adds the value of an ECPT meta field to the post class

meta_field_class.php

function ecpt_meta_field_classes($classes) {
    $meta_value = get_post_meta(get_the_ID(), 'ecpt_fieldname', true);
    if($meta_value) {
        $classes[] = $meta_value;
    }
    return $classes;
}
add_filter('post_class', 'ecpt_meta_field_classes');