↳ In-reply-to » I'm thinking of a kind of 'boost' or 'retweet/repost' for twtxt, so it can help with discoverability of interesting people to follow.

What about using the blockquote format with > ?

Snippet from someone else’s post
by: @eapl.me@eapl.me

Would it not also make sense to have the repost be a reply to the original post using the (#twthash), and maybe using a tag like #repost so it eaier to filter them out?