Force Drush8 to Download Drupal 8 Module on a Remote D7
I was going to suggest --default-major. Since that does not seem to work, I can only recommend to download the module outside of a Drupal site and then move it into place. 

drush dl composer-7 --destination=/tmp &amp;&amp; drush rsync @self:/tmp/composer @remote-dev:sites/all/modules