Ha! I have been working on the same thing this weekend, except it uses the rss feed for posts and scrapes old.reddit.com for the details. It's written in python, but not quite finished - scraping works, automation not yet.
My plan was to have a separate Lemmy instance for this, where people can also request for new subs to be included. This would reduce the spam in bigger communities, and allow instances to block it all together if they wanted to.
Beside that, I'd pre- or postfix each post with a message it's a copy and a link to the original for copyright reasons. Moderation would be a separate story - Not particularly looking forward to that. Could make it so that if a post were flagged, it would re-aync with the original. Let reddit do the moderation :D