Ah, I’d missed this question here (but I answered it on XMPP where I was also asked).
You did answer in the chat room, it's fine!
I guess this isn’t relevant now, but I wonder if it would work better as the user’s server announcing the unavailability (and re-availability) of the remote, in a more PSA-ish way, if done within PSA.
If we follow XEP-0045, unavailability is sent to each occupant already, so I guess we would need to change that too? Implementations in the wild already do not necessarily announce that they go down, though. The MUC service could announce its re-availability to the remote user's directly, but then the server has to know users it has to dispatch the announcement too. I can imagine various ways to do that, maybe PSA is a good fit for this traffic optimisation. -- nicoco