Allow modifying the set of allowable transitions

FYI, here is my implementation of the new alter hook:

/**
 * Implementation of hook_workflow_allowable_transitions_alter().
 */
function mymodule_workflow_allowable_transitions_alter(&$transitions, $sid, $dir, $roles) {
  // List of 'to' transition states to check.
  $sids = array(2, 3);

  if ($dir == 'to' && in_array($sid, $sids)) {
    // Get the node object for the current page (first param set to FALSE
    // means we don't have to be on a node view page, can also be on e.g. a
    // node editing or node workflow page).
    $node = mymodule_get_curr_page_node(FALSE);

    // If we're on a node page, and user is not a global admin, and user
    // is also not an OG admin for the current node, make certain transitions
    // unavailable.
    if (!empty($node->nid) && !mymodule_check_role_access('administer nodes', $roles) && !og_is_group_admin($node)) {
      // List of 'from' transitions to unset.
      $unset_tids = array(4, 7, 8, 11);

      foreach ($unset_tids as $tid) {
        unset($transitions[$tid]);
      }
    }
  }
}

Oops, my hook implementation above doesn't work (teach me to test before posting snippets!). og_is_group_admin() needs to be passed the node of the group, not the node of the current page (which is assigned to the group). Once again, this is just FYI, for people who want to achieve the same OG to Workflow integration that I've set up:

/**
 * Implementation of hook_workflow_allowable_transitions_alter().
 */
function mymodule_workflow_allowable_transitions_alter(&$transitions, $sid, $dir, $roles) {
  // List of 'to' transition states to check.
  $sids = array(2, 3);

  if ($dir == 'to' && in_array($sid, $sids)) {
    // Get the node object for the current page (first param set to FALSE
    // means we don't have to be on a node view page, can also be on e.g. a
    // node editing or node workflow page).
    $node = mymodule_get_curr_page_node(FALSE);
    if (!empty($node->og_groups) && is_array($node->og_groups)) {
      reset($node->og_groups);
      $og_nid = key($node->og_groups);
      $group_node = node_load($og_nid);

      // If we're on a node page, and user is not a global admin, and user
      // is also not an OG admin for the current node, make certain transitions
      // unavailable.
      if (!empty($group_node->nid) && !mymodule_check_role_access('administer nodes', $roles) && !og_is_group_admin($group_node)) {
        // List of 'from' transitions to unset.
        $unset_tids = array(4, 7, 8, 11);

        foreach ($unset_tids as $tid) {
          unset($transitions[$tid]);
        }
      }
    }
  }
}