Struggling with the same issue at the moment. It seems that placeholders are scattered everywhere through the plugin files making it a little difficult to reproduce.
Edit: Alright, it seems that there might be an easier way though I’m sure you might need to keep looking for something better. Go to the events-manager/classes folder, and open em-booking.php. In the output function you can add your own placeholder. Something along the lines of:
case '#_USERMOTORCYCLE':
$replace = $this->get_person()->user_motorcycle;
break;
That should do the trick for now.