On one of the issues about quote posts Mike Macgirvin shared the insight that quote posts can already be done in AP without extentions. "We have attachments. Set the attachment mediaType to an ActivityStreams media type and let clients decide whether or not or how to format and display it."

I really like this idea honestly and I'm seriously considering trying this out.

I'm not really sure, but I guess it could look something like

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "type": "Note",
  "content": "Check this post out!<br>RE: https://example.org/notice/sqdfqsfd";,
  "attachment": [
    {
      "type": "Note",
      "mediaType": "application/ld+json; profile=\"https://www.w3.org/ns/activitystreams\""
      "url": "https://example.org/notice/sqdfqsfd"
    }
  ]
}

But if I understand correctly, they already do this for Zap, so I should first check if and how they do it there.