Spite-Driven-Development continues.
I now have a scrap of semantic HTML which adequately replicates an embedded Tweet.
TODO:
* Add images
* Add entities (hashtags, mentions, links)
* Add parent of thread
* Add quoted tweet
* Add ... whatever else
Should be able to hijack the embed API to get most metadata (no number or replies / retweets though).
Parents and Quotes are just <blockquotes> within <blockquotes>. So that was easy!