Is possibe use sprite on carrierwave fallback?

I'm using carrierwave for upload images and to reduce requests i'm putting everything as possible on sprite. But i came with the Carrierwave fallbacks.

I would like to know if have a way to indicate the Carrierwave the fallbacks is on a sprite.

Since i never saw any documentation about this.

  def default_url
    '/assets/fallback/' + [version_name, 'avatar.png'].compact.join('_')
  end

thank's