Why not just change that? You are already in the same host, you can write the port that the server bound to on startup to a file and just read that from the client.
I've implemented something similar and went through the same stage "I will wait until someone complains" and they will happen, just prevent them right now :)
But for now we just hard-code a port. I personally prefer this since it's easy to use and convenient to remember. But if a lot of client machines have conflicts, I guess we'll change that...