Proposal is still under discussion for some of the technical details, but I expect it to be formalized at one point. I also want to spend some time to see if I can write tests for the context.
Generally, it’s easy to extend the json-ld @context and thus the vocabulary. One can simply add terms. Mastodon does this by just inserting a big dictionary into the @context.
I unfortunately do not understand what you mean, or your examples following it. Maybe you mean something else than I do with a document. I would mean some type of json-ld object having an id. What do you mean?
You mean inline syntax, right? Just wanted to mention the Agora bot here. You follow it, and then it parses your toots for hashtags, but also wiki-like markup such as [[some term]], which it transfers to a shared knowledgebase.