Session regotiation option

We have a networking application that we would like to secure (authentication, 
encryption, and integrity) using port forwarding feature of SSH. In some rare 
customer setup where they have a private trusted network, we would still like 
to use ssh for user authentication, port forwarding to connect to the 
application. However, we would like to switch to none cipher & mac mode after 
authentication is done if this is possible.
 
We understand that the primary purpose of SSH is to provide a secure 
session/tunnel, but in some instances, our application can run on a slow 100Mhz 
micro-controller and we would like to avoid the overhead of encryption & mac if 
possible.
 
My questions are follows:
* Does SSH protocol have an option to re-negotiate the ciphers during a session 
(just like SSL/TLS)? 
* If so, does OpenSSH client support this option if our SSH server sends a 
renegotiate option during a session? 
* Any other suggestions to switch to plain security mode after client 
authentication (password based authentication)
 
Thank you very much and regards,
-Sanjeev Sood