Is there any way to add comments to pages which are NOT nodes
API the module that handle the pages you see at api.drupal.org, has the same problem; the documentation pages you see are not nodes, but the module allows to add comments to them.  

The page callback invoked for the documentation pages is api_page_function(), which is associated to the path api/%/%/function/%api_item in its implementation of hook_menu() (see api_menu(), starting at line 125 of the linked file). The page callback uses _api_add_comments(), which contains the following code:

if (module_exists('comment') &amp;&amp; user_access('access comments') &amp;&amp; variable_get('comment_api', COMMENT_NODE_READ_WRITE) != COMMENT_NODE_DISABLED) {
  $output .= comment_render(node_load($documentation_object-&gt;did));
  if (user_access('post comments')) {
    $output .= comment_form_box(array('nid' =&gt; $documentation_object-&gt;did), t('Post new comment'));
  }
  // ...
}


API creates empty nodes that associate to the documentation pages; the reason of this is that the Comment module still need a node object to return any comment that is associated with that node.