Validate an ubercart checkout pane?
Would that checkout pane still be captured by hook_alter_form()?  If so you could add #required to the field there. Let me have a quick try and I'll get back to you.

EDIT: OK, this worked for me:

function tools_utilities_form_alter(&amp;$form, $form_state, $form_id) {
  if ($form_id == 'uc_cart_checkout_form') {
    // Make delivery date required
   $form['panes']['ddate']['ddate']['#required'] = true;
 }
}


Create your own module (mine is called tools_utilities in this example) and implement the above and you should be good to go :)