We have no actual plans to implement federating mechanisms, but we have the will to find a way to do it at some point.
I wonder, though, if SMTP would not be a better protocol for this than web-based stuff… Here are my current thoughts.
A seenthis message would be an email message, with Message-Id: ${UUID}, In-Reply-To: ${UUID}, a MIME text part (the raw text for your database), an HTML part (processed message for your eye pleasure).
Implementing “updates” would just be a resend either with the same Message-Id (but this might be illegal), or as an in-reply-to with a secondary header like “Update: (previous message hash or something)”
This would allow a user to send emails to seenthis, too, and to reply by email to a conversation. (I don’t see an essential difference between these worlds.)
You would then simply store data in an IMAP store, and so on…