Hi Will,
The helper function can get the field's value in the admin area that is stored in the custom table. But you need hooking to init with the higher priority such as 99 or a hook fires after init to make sure the table and meta boxes are registered. Here is the sample code:
add_action( 'init', function() {
$field = rwmb_meta( $field_id, ['storage_type' => 'custom_table', 'table' => $table], $post_id );
echo "<pre>";
print_r($field);
echo "</pre>";
}, 99 );
For more WordPress Hooks Firing Sequence information, please follow this article http://rachievee.com/the-wordpress-hooks-firing-sequence/.
.
