There’s an option in the wp_sitemeta table with meta_key ‘menu_items’
this value is referred to the checkbox “Administration menu” of the page /wp-admin/network/settings.php (accessible only to siteadmins) that allows the activation of plugins on single sites.
I’ve not tested but using the value a:1:{s:7:"plugins";s:1:"1";}
and having the plugin uploaded via ftp you should see the plugin page on the site you’ve access to..