Attempt to nest Drupal 7 FAPI element type tableselect (Note: It doesn't work!)

megaselect.module

<?php

/**
 * @file megaselect.module
 * TODO: Enter file description here.
 */

/**
 * Implements hook_menu().
 */
function megaselect_menu() {
  $items['megaselect'] = array(
    'title' => 'Megaselect',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('megaselect_megaform'),
    'access arguments' => array('access content'),
    'type' => MENU_CALLBACK,
  );

  return $items;
}

function megaselect_megaform($form, $form_state)
{
  $users = array
  (
    array('uid' => 1, 'first_name' => 'Indy', 'last_name' => 'Jones'
      , 'nicknames' => array('indy', 'andy', 'pandy')),
    array('uid' => 2, 'first_name' => 'Darth', 'last_name' => 'Vader'
      , 'nicknames' => array('durth', 'dearth', 'dark')),
    array('uid' => 3, 'first_name' => 'Super', 'last_name' => 'Man'
      , 'nicknames' => array('supper', 'dessert', 'stormy', 'catcherman')),

  );
 
  $header = array
  (
    'first_name' => t('First Name'),
    'last_name' => t('Last Name'),
    'nick_name' => t('Nick Name'),
  );
  $options = array();
  foreach($users as $user)
  {
    $rowid = $user['uid'];

    $inneroptions = drupal_map_assoc($user['nicknames']);
    $innerheader = array('Choose a Nickname');
    $innerform = array (
      '#type' => 'tableselect',
      '#header' => $innerheader,
      '#options' => $inneroptions,
      '#multiple' => FALSE,
      '#title' => 'Nickname',
      '#title_display' => 'invisible',
      '#default_value' => '',
      '#id' => 'nickname_row' . $rowid . '',
      '#name' => 'nickname[row' . $rowid . ']',
      '#empty' => t('No nicknames to choose from'),
    );

//    dpm($inneroptions, 'inner options');

    $options[$rowid] =array
    (
      'first_name' => $user['first_name'],
      'last_name' => $user['last_name'],
      'nick_name' =>  array('data' => array(
          $innerform,
        ),
      )
    );
  }
  $form['table'] = array
  (
    '#type' => 'tableselect',
    '#header' => $header,
    '#options' => $options,
    '#empty' => t('No users found'),
  );
  $form['nickname'] = array(
    '#type' => 'value',
  );
  $form['submit'] = array
  (
    '#type' => 'submit',
    '#value' => t('Submit'),
  );
  dpm($form);
  return $form;
}

This doesn't work (yet?). The example whipped up is based on the clever hack on how to stuff more FAPI form fields into a tableselect than are allowed out of the box (none). I am not sure it can be done... but I'd love to be proven wrong!