In .ssh/config:

    Host alias
        Hostname realname.domain
        Port alternate_port

I've browsed through the site and installed to look at the man page to find this out. I can't see a way to do this. Makes it useless for me :-(

Not that I'm complaining. It's not like I'm being asked to pay for it, just a feature that I imagine would be helpful to many people...

A workaround for this is to change the ssh default port in your ssh config file.