If drush ends up using composer as the recommended install method, then composer could be this tool, provided terminus can somehow be hosted on packagist (another matter entirely):
https://github.com/drush-ops/drush/pull/129

    composer global update --dry-run

Hoping composer maintainers are in support of a porcelain command that would make this even simpler:
https://github.com/composer/composer/pull/210#issuecomment-24893674