Good news, since version 3.1.6 Really Simple SSL supports WP-CLI for basic SSL management. This version is not live yet in the WordPress repository and can be found on the Really Simple SSL Github page. The Really Simple SSL commands can be accessed with wp rsssl. The plugin currently supports two basic commands: one to activate SSL (activate_ssl) and one to deactivate SSL (deactivate_ssl).
We would like to extend the plugins WP-CLI functionality with your suggestions. Please us know what functionality you would like to have included in the Really Simple SSL WP-CLI commands!