Doable. It would basically be ssh but without encryption. You'd have to bang out login by hand, but "username\npassword\n" will probably work, might need a sleep inbetween, and of course you'll have to detect successful login too. Oh, and every 0xff byte will have to be escaped with another 0xff

At that point, may as well support raw serial too.

Supporting rlogin on the other hand is probably as simple as GIT_SSH=rlogin