<p>I have a small local network. Only one of the machines is available to the outside world (this is not easily changeable). I'd like to be able to set it up such that ssh requests that don't come in on the standard port go to another machine. Is this possible? If so, how?</p>

<p>Oh and all of these machines are running either Ubuntu or OS X. </p>

You can use Port Fowarding to do this. Take a look here:
http://portforward.com/help/portforwarding.htm
There are instructions on how to set up your router to port forward request on this page:
http://www.portforward.com/english/routers/port_forwarding/routerindex.htm