Edit a node via code authentication
Here is a solution using custom coding.


Store all hash codes that are the access code to the DB (I think you do this already), a word of caution do not do md5(node id) or sha1(node id) as anyone will be able to figure it out.
Enable anonymous users to edit the content type they are submitting where the code is needed to edit the node. I know what this mean, keep reading.
Create a custom module where you are going to use the NODE API to intercept the node edit step.


If Drupal 6:  http://api.drupal.org/api/drupal/developer%21hooks%21core.php/function/hook_nodeapi/6

function YOUR-MODULE-NAME_nodeapi(&amp;$node, $op, $a3 = NULL, $a4 = NULL) {
  switch ($op) {
    case 'prepare':

    // Add PHP Code here to validate the hash code
    // You can use an if/else condition to print an access denied message
    // if the code is not valid or expired

    break;
  }
}


If Drupal 7: http://api.drupal.org/api/drupal/modules%21node%21node.api.php/function/hook_node_prepare/7

function YOUR-MODULE-NAME_node_prepare($node) {
    // Add PHP Code here to validate the hash code
    // You can use an if/else condition to print an access denied message
    // if the code is not valid or expired
}


And that is all.