When SSH clients first connect they ask the server for a pseudo terminal (PTY), to enable terminal colors, clearing the screen etc.
In this case the SSH server declines that request because it simply wants to send back one line of text. The client falls back to text mode and works fine, but issues the warning anyway. You can suppress it with `ssh -T mars.vtllf.org` like I did in the demo video.
I definitely echo the sentiment of wanting to use these sorts of power-user-only tools. But I think Moxie's recent post, They Live[1], does an excellent job of explaining why we shouldn't accept that the tools are too hard for everyone else, then just hoard them for our own use anyway.
Good point about the <token>@<host> format, I like it much better too.
You're right about tying tokens to host names, that would work. But you'd have to justify whether the extra complexity of multiple web servers preregistering tokens with a SSH front-end was worth it. Another approach would be embedding a static host identifier in the username as well.
Although my knowledge of the SSH protocol isn't complete, a related issue appears to be that servers prove their identity before clients send their usernames. That means servers sharing the same IP/port would also have the same host fingerprint.
It could be acceptable if all the services were run by the same organisation, but on a platform like Heroku it would be more of a challenge.