demonstrates a simple filter for capturing a dynamic value on the admin side in Participants database

pdb-backend-dynamic-values.php

<?php
add_action( 'pdb-form_element_build_text-line', 'xnau_place_dynamic_value' );
/**
 * sets the value of a dynamic hidden field in the backend
 * 
 * @param object  $field
 */
function xnau_place_dynamic_value( $field )
{
  /*
   * the field must be a regular pdb field and we must be in the backend
   */
  if ( isset( Participants_Db::$fields[$field->name] ) && is_admin() ) {
    $default_value = Participants_Db::$fields[$field->name]->default;
    /*
     * don't change the value if it's already set
     * check that it's a dynamic field
     */
    if ( empty( $field->value ) && Participants_Db::is_dynamic_value( $default_value ) ) {
      $field->value = Participants_Db::get_dynamic_value( $default_value ); // set the value
    }
  }
}