At https://secure.phabricator.com/book/phabricator/article/diffusion_hosting/ it suggests several ways to deal with the fact the git-ssh server can't (easily) be on port 22. It says: "The easiest way to fix this is to put a load balancer in front of the host and have it forward TCP traffic on port 22 to port 2222."
But I think there's a way that may be easier in practice, which is what we do: use .ssh/config to specify the port. The way it works is you set up a CNAME to point to your host, for use by git (git.myorg.com or phabricator-git.myorg.com or whatever). Then you have all your users put this into their .ssh/config:
Host phabricator-git HostName phabricator-git.myorg.com Port 2222
Then they can do git clone git@phabricator-git:<repo> and it automatically uses port 2222.
I'd suggest adding this to the documentation, maybe as the first suggestion. It does require all users to take some action, but is easier to set up than the alternatives.