Allow users to label their own comments with "spoiler" and provide a text input (similar to Exemplary), that all users can then see, to indicate what the spoiler pertains to.

As discussed here:
https://tildes.net/~tildes/d42/spoiler_tags_should_also_display_on_comments_in_tagged_posts_when_viewed_from_a_user_page#comment-36gn