You are all over thinking this whole thing. Just go to you PS3 and give it an incorrect gateway address.
1. Access the Network Settings from the XMB and change your settings from "Automatic" to "Manual".
2. Assuming that you are on a 192.168.1.X network, give your PS3 a network address of 192.168.1.25 with a subnet mask of 255.255.255.0.
3. Assign the Gateway as 192.168.1.50.
4. Assign the DNS Server as 192.168.1.50.
You can now access your PS3 via FTP from your PC by connecting your FTP client to 192.168.1.25. Your PS3 will be able to access your local network but not the internet and you don't have to do anything special to anything else on your LAN.
You can't use 192.168.1.255 as an IP address as that is a broadcast address.