These all seem
like things an XMPP server could choose to support. I don't
think there's any standards work blocking such an implementation.
It could be implemented, but wouldn't it require breaking
compliance with XEP-0045 by introducing a new send-only role, cf
<https://xmpp.org/extensions/xep-0045.html#table-3>?
You don't need a new role in order for the server to treat a member
differently. Role is just outward facing metadata vaguely sunmarizing *part*
of the internal state.
A more complex
set up would be partially allow them to read messages directed to them and exclude the
rest of messages.
MUC PMs would work for this, wouldn't they?
Yes. Or could do as I do in muc_adhoc_bots module and "install on the room"
so there is no participant at all but eg adhoc commands can be added to the
room. This is probably a good fit for most send-only sttle bots and works
already today.