you can setup port forwards on-the-fly with ~C

~? shows the few things you can do over the admin channel of ssh.

  ~ $ ~?
  Supported escape sequences:
    ~.  - terminate connection (and any multiplexed sessions)
    ~B  - send a BREAK to the remote system
    ~C  - open a command line
    ~R  - Request rekey (SSH protocol 2 only)
    ~^Z - suspend ssh
    ~#  - list forwarded connections
    ~&  - background ssh (when waiting for connections to terminate)
    ~?  - this message
    ~~  - send the escape character by typing it twice
  (Note that escapes are only recognized immediately after newline.)