Hmm. Sounds like you probably know more than me…but ActivityPub uses ActivityStreams 2 as its data model, which has a Mention object type, and Mastodon includes those Mentions in the tag field of its objects, right?

For example, if you fetch the AS2 version of your reply with Accept: application/activity+json, it has this tag:

{
  "type": "Mention",
  "href": "https://mastodon.social/users/mcc",
  "name": "@mcc@mastodon.social"
}