SMB proxy: Connect to SMB server through SSH tunnel between two other hosts

The answer that @Zoredache gives to this question is a good one for what you're looking to do, though I'm going to elaborate on it for your situation.

It's not clear to the if the router is running an SSH server from your diagram or if you're just saying that you're going to be moving SSH traffic through it. I'm going to assume you're just moving SSH traffic through it via a port-forward, in which case I'd:

• Start an SSH session from your PC to the public IP address of the router which, presumably, has a port forwarded to the Debian VM's SSH server port

• Forward port 139 on your PC, specified as 127.0.0.2:139, to the LAN IP address of the Windows 7 PC, port 139 (this assumes the Debian VM can access the Windows 7 PC). If you're using OpenSSH the syntax would be ssh router-ip -L 127.0.0.2:139:win-7-pc-ip:139

• Create a /etc/hosts (or whatever analog to that exists on your OS) to resolve the name of the Windows 7 PC to 127.0.0.2

This will allow you to access the Windows 7 PC, by name, via a local loopback address that forwards to the SMB port on the Windows 7 PC.