It can be attached to whatever you like - the important thing is that you render it.
So you could quite simply use:
$foo['#attached']['css'] = drupal_get_path('module', 'MODULE') . '/MODULE.css';
Or stick it in a var like you're already doing:
$variables['SOMETHING']['#attached']['css'] = drupal_get_path('module', 'MODULE') . '/MODULE.css';
And in the template:
It works because
drupal_render() unconditionally calls
drupal_process_attached() on any element it finds with an
#attached property, regardless of what you're doing with the result of the render (or if there's even anything to render).