You could also use phex and tell it to cennect to a differently named network. (switch network).
Only Phex clients who know the network name will then be able to connect. What exactly it does in a LAN, I doN't know (there is an option to connect to the LAN, which is standardly enabled. I don't know, if only setting this and not telling it to connect will only connect to clients in the LAN).
It still uses GWebCaches.
Phex is written in Java. so usable on Mac and PC, I use it myself in OSX.
http://phex.kouk.de
You might want to ask the developer to create a special version for you in the phex forum:
http://www.gnutellaforums.com/forumd...?s=&forumid=16