That is a bad idea. Cross posts aren’t a special type of post. Your instance simply checks the URLs of posts and finds other posts with the same URL. That is how cross posting works in Lemmy.

https://github.com/LemmyNet/lemmy/blob/6f45f801cc0582b14360fd52737cbad3a98b7250/crates/api/api_crud/src/post/read.rs#L90

[...]

  // Fetch the cross_posts
  let cross_posts = if let Some(url) = &post_view.post.url {
    SearchCombinedQuery {
      search_term: Some(url.inner().as_str().into()),
      post_url_only: Some(true),
      type_: Some(SearchType::Posts),
      ..Default::default()
    }

[...]

Piefed seems to do the same thing

[...]

new_cross_posts = db.session.query(Post).filter(Post.id != self.id, Post.url == self.url, Post.deleted == False,
                                                        Post.status > POST_STATUS_REVIEWING).order_by(desc(Post.id)).limit(limit)

[...]

I also found this weird hardcoded ap id in the function that “calculates” cross posts in piefed for some reason:

[...]

if self.community.ap_profile_id == 'https://lemmy.zip/c/dailygames':
            # daily posts to this community (e.g. to https://travle.earth/usa or https://www.nytimes.com/games/wordle/index.html) shouldn't be treated as cross-posts
            return


[...]

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