How can I dynamically change the display format?
You can do that printing directly the entityreference field in the node.tpl.php. First you need to check if the image field exists and then display the entity reference, so you can do something like this:

if(!$content['field_image']){
   //Load default image
}else{
   print 'Your custom label.';
}


You have to remember to set the format to hidden in the display section.