Use NFS instead of FTP.
1) Create the bootdisks. I'm not sure about 9.0, but for the beta you needed bare, install1, install2, and network.
2) Use the bootdisks.
3) Once you login as root, type: network. It'll search for the network disk and prompt you for it. Put it in and hit enter. Hit enter again to scan for your ethernet card. Once it's finished hit enter again and you can remoe the disk.
4) Make sure the "other machine" has nfsd running and the directory that has slack on it is in the /etc/exports file.
5) Install on the laptop like normal selecting NFS as the install source. You'll probably have to specify an IP address for the laptop as well as for the nfs server.
That should work for you. I've done this several times and this method is my favorite.