Using "Additional Validation" for complex conditional recipients?

I've got a form where a user enters their zip code, and I want to send the form to whichever employee covers that zip. The standard Conditional Recipient select list doesn't really work here because there are way too many zips for one list, and also because when a single zip is covered by more than one person, I want to randomly select which person gets the lead.

So, what I was thinking is to do a bunch of PHP processing in the "Additional Validation" section based on zip (which is $form_values['submitted'][2] in my form), then re-set the hidden email field to be the appropriate address. I figured that then the "validation" would complete and the form would be processed like normal and would send off to the new address.

$zip = $form_values['submitted'][2];

// do a bunch of stuff based on zip code, then eventually get to this: 
		if (in_array($zip, $zip_code_list)){
			$form_values['submitted'][4] = $new_email_address;
			$form_values['submitted_tree']['email_address_send'] =$new_email_address;
			break;
		}

The processing works perfectly, and if I do a print_r($form_values) before and after this code, the email field shows having the correct address in place after the processing. But when I go and look directly at the submissions, all of the entries show the default value in the email field.

Is there some other value I should be changing, or somewhere else I can put this code that will make it change the value of that field (so that the email will get sent to the right person)?

Thanks!