Assuming that Comment belongsTo many commentable models (Article being one of them), you could do:

class Comment extends AppModel {
  var $actsAs = array(
    'NamedScope' => array(
      'promotedArticles' => array(
        'Comment.model' => 'Article',
        'Article.promoted' => 1
      )
    )
  );
}

Such scope would find all comments to promoted articles.

Scope’s conditions are copied to find query without any modifications so it is OK to have multi-model conditions in Comment’s model named scope.

Of course you still need some support for polymorphism before. I believie that PolymorphicBehavior or something similar would be fine.