Theme and Override Privatemsg messages/view/%thread
Take a look at privatemsg-view.tpl.php. 

There is a preprocess hook where you may add theming variables to the privatemsg-view.tpl.php and execute any PHP code there. In my case it was: 

&lt;?
function MYTHEME_preprocess_privatemsg_view(&amp;$vars) {
  $vars['author_uid'] = $vars['message']-&gt;author-&gt;uid;
  $vars['realname']= MYMODULE_get_realname($vars['message']-&gt;author);
}
?&gt;


So here are some alternatives to populate the sidebar:


Use this PHP code to trigger a drupal context which will show a block on the sidebar
Programtically create other vars like the ones above. Those vars may contain sidebar data  you can use when theming the TPL file which contains the sidebar.


Also, here is a full privatemsg theming guide.