There is no host discovery service in Gnutella today that can be limited to a LAN.

It would require a protocol extension and servent updates to be able to do what you want. Basically, you would set up a private UHC and then configure servents to bootstrap using that UHC only. All servents would be configured to only connect on the LAN address segment(s) and the UHC would be also configured to accept connections from this LAN topology and would only cache hosts lying in that LAN.

The architectural elements are there in Gnutella today, but there is no configuration of these elements possible today to achieve your goal.

You can get where you want a little bit quicker if you only limit yourself to one single Gnutella vendor and manage to specify exactly what you want so that proper configuration and support can be added by that Gnutella vendor.