jQuery Validate: How to validate a non-form element?

Question

I have a form that has an attached map. Clicking on the map adds hidden inputs to a div in the form.

I want a validation rule to check whether or not that div has any hidden elements. So far, I've only been able to do this with a dummy element that I have to manually remove before posting the form:

<form action="..." method="post" id="signup">
  <!-- ... -->
  <div id="zones_selected"><input type="hidden" name="dummy"></div>
  <!-- ... -->
</form>

$.validator.methods.zones = function (value, element, param) { return ($zone_holder.find('input[name!=dummy]').length > 0); };
$('#signup').validate({
  rules: {
    //...
    'dummy': 'zones'
  },
  //...
  submitHandler: function(form) { $(form).find('input[name=dummy]').remove(); form.submit(); }
});

However, these leaves much to be desired for error placement, as my custom validator method doesn't seem to ever trigger the unhighlight function, and I feel dirty for having put in an input solely for form validation (not very unobtrusive).

What I would like is no extra submitHandler and a rule to check the presence of hidden inputs in a div that would have an unhighlight trigger (such that if a user does click on the map--which creates a new hidden input--the message I display telling them to do so will disappear).

What's the best way to accomplish this?