I split my initial post in a number of issues on the new activitypub-interop repo, so that it will be easier to discuss the details. Feel free to create other issues, or comment the existing ones!
Only some testing and research. No meaningful code, yet. This is still high on my list, but I was busy with other work.
I looked at how json-ld looks when it’s produced by CSS, and if it looks anything like the simple format that e.g. Mastodon may expect. (it does not.) There could be a way - having the webId stored as json-ld; but it’s already causing issues - NSS doesn’t authenticate such agent (i suspect it doesn’t ask for text/turtle, and receives the default json-ld instead). CSS seems fine with webId stored in json-ld representation.
Also thought of a name for the project, came up with soap-opera