The main issue here is that the node reference widget has no idea of the zip code field because it hasn't been submitted into the database yet. Yes, you have typed information into it, but until the form is submitted, it's still just out there with the client.
What you will probably have to do is code up some custom AJAX/AHAH for your form to look when your user has entered information into the zip code field, then pass that to drupal and then have drupal return your node information based upon that filter (See Views) and either repopulate or initially populate a drop down list of nodes.
Lots of information on this can be found in the Drupal Form API Reference and most probably some code examples in the aptly titled Examples Module.
Side note: If these were all node references, you could also investigate the Hierarchical Select Module.