Autopopulate "Postal Code" from taxonomy in Location module
I think the only way to do this is with a custom module.

You would implement hook_form_alter to disable (or hide altogether) the location postal code on the node form.

You would then implement presave op of hook_nodeapi (drupal 6) or hook_node_presave (drupal 7) to fill in the location data.

If you don't know PHP, you could also add some custom javascript that changes the location postal code when the taxonomy drop down is changed.