my understanding is that if you change the connect string and connect OK strings to something else then yes you can create an isolated network. Gnutella clients check that string and if it matches theirs then they can connect to each other. So if you change it so your connect string is:

LAN PARTY

and your connect ok string is:

LAN PARTY OK

then everyone who want's to connect to it must have the same strings.

However if you want the lan party to work you'll need everyone's IP's and then you can either post them like the gnutella host cache's out there and have people connect to your host cache's or you need to distribute the IP's to everyone (and I'm gonna guess that not everyone is going to go for the second one.) So it may be a bit of work but if you could work it so people sign into the cache host and log their IP on it or something it may work out not bad... good luck