I've been using the html hidden type for a long time but never stopped to consider what type of data the field was ment to display until now when I need to preset hidden input fields to be auto-saved to the database. I was confusing a display type with an actual data format type used by the database. A hidden form field is just that, Hidden. It needs no display formatting.
Unfortunately the c5 form helper does not take additional parameters for $form->hidden(), so its not so easy to, for example, add css classes for convenience in jQuery selectors or fix the max size.