Restricting ports wouldn't be easy with phex, as every client sets a random port at first run.
You could setup some GWebCaches (
http://www.gnucleus.com/gwebcache/ or
http://home.comcast.net/~jjulian16/ ) for which you set "supported networks" to the name of your college-network, download the phex-source (
http://sf.net/projects/phex ) and replace the preinstalled list of GWebCaches (simple text-list) with your networks ones (which should best be set to allow only connections from the College-LAN, but with a bit tweaking they won't allow connections from non-college-clients at all, and your connections won't see IPs of clients of another network whatever you do).
After compiling it, you can safely distrubute it. (You only need the GWebCaches for the first connection, then the program manages a local list of clients (the HostCatcher).
Still I would like to try a Mac-Port of Gnucleus, if you can do it. (You might want to pay a visit to
http://gnucula.sf.net )