SSH connect to a UNIX socket instead of hostname

You should be able to do utilizing socat and ProxyCommand option for ssh. ProxyCommand configures ssh client to use proxy process for communicating with your server. socat establishes two-way communication between STDIN/STDOUT (socat and ssh client) and your UNIX socket.

ssh -o "ProxyCommand socat - UNIX-CLIENT:/home/username/test.sock" foo

I've submitted pull request to make this work, but it haven't been merged yet:

https://github.com/openssh/openssh-portable/pull/162