I was under the impression that adding
to in a CiviMail would log someone in to the dashboard but it does not seem to be working for me.
This answer also suggests it so maybe I am doing something wrong.
PS I also tried id in place of cid (see below) but it did not work either.
PS https://github.com/civicrm/civicrm-core/blob/master/CRM/Contact/Page/View/UserDashBoard.php does not mention checksums so i am guessing it is not possible...