@ink As there would be a distinction between the protocol level and the clients implementing it, I'm thinking of approaching that like so:

The protocol level allows including the 'speaker' with each message. The client is responsible for sending along that tag, and exposing a sensible mechanism (like 'sessions') to let the user control it.

If no tag is specified, the speaker will be the default/fallback identity for the account, which in a plural setup would be eg. an account representing the system/collective/etc. as a whole.

Would that be sufficient?