Sadly, I think you're going to be chasing your own tail. The socket is owned by root... because sshd is running as root. The user is not established until AFTER the user has authenticated... but since the connection is never closed, the socket is still owned by root. Short of trying to do some sort of reverse-matching between the socket ID and the current user after the fact... I don't know how you would accomplish this. I have seen custom hacks directly into the openssh src that add some level of Traffic Accounting... but they are highly version specific, and I doubt you'll see one ever make it into the standard repository.
I found a hack-ish example of accounting after-the-fact. I question it's reliability, and accuracy... but it's better than nothing. Basically, it relies on your auth.log reporting on users' connection states. (i.e. connects/disconnects) and
tail running continuously. Unfortunately, tail has a limit of 1-2 seconds minimum between updates... and there is a margin of error when it comes to monitoring the auth.log. (especially during log-rotates)