I'd like to do the following with Python:

Computer 1 starts SSH server (probably using twisted or paramiko)
Computer 1 connects to Server 1 (idle connection)
Computer 2 connects to Server 1
Server 1 forwards Computer 2's connection to Computer 1 (connection no longer idle)
Computer 1 forwards Server 1's connection to listening SSH port (on computer 1)

Result being Computer 2 now has a SSH session with Computer 1, almost as if Computer 2 had started a normal SSH session (but with Server 1's IP instead of Computer 1's)
I need this because I can't port forward on Computer 1's network (the router doesn't support it).
I'd use ssh to create a remote tunnel (-R) from the server to the local system. If you're insistent on doing this with Python then there's the subprocess module.
Establish SSH Connection Between Two Isolated Machines Using a 3rd System