$ dns-sd -P "any name" _daap._tcp. local 3690 localhost 127.0.0.1 &
$ ssh -N user@server.example.com -L 3690:localhost:3689
Replace user with your short username on the remote Mac, and server.example.com with the public IP of the remote Mac. Don't forget to set up your remote router so that the ssh port (22) is forwarded to a Mac on which the SSH daemon is launched (Remote Login enabled on the Sharing System Preferences panel).
You can put this in a shell script (thanks to andersB):
#!/bin/bash
dns-sd -P "name of server" _daap._tcp local 3690 localhost 127.0.0.1 &
PID=$!
ssh -N user@server.example.com -L 3690:localhost:3689
kill $PID
This will start the broadcaster, set up the tunnel, and kill the broadcaster once the tunnel closes. I use port 3690 on the local machine, as to not interfere with the iTunes sharing on that host. This enables me to both see the remote share and share something from my local iTunes with the users on the network your laptop is in. It could be perfect if someone could find a way to double-click this script to run it.
[robg adds: You should be able to run this from the Finder by just double-clicking the shell script file; on my 10.5 machine, at least, that opens Terminal and runs the script. Alternatively, you could paste the script into an Automator action and save the result as an application. Note that I haven't tested this hint.]

