I'd be glad to submit a PR to do this but I'd like to ask: how should this be represented? As an Article? As a Note with hidden content? How would the content be hidden? Should the extended content be federated at all?

Looking at the spec again, using the summary field might be plausible: the ActivityStreams Summary field (https://www.w3.org/TR/activitystreams-vocabulary/#dfn-summary) says it is to be interpreted as HTML... how does Mastodon treat this, or older versions of Calckey? How about Akkoma?