Webform Custom-Field Module and validation
It looks like the hook _webform_action_set_component might be used for validation: "Setting the form element allows form validation functions to see the value that webform has set for the given component."

http://www.rit.edu/drupal/api/drupal/sites!all!modules!webform!webform.api.php/function/_webform_action_set_component/7.41