There are a couple of ways I could think of, but I have found it is a bit of a problem problem for the average Limewire user to find what they are loking for and get stable downloads, and anything I could suggest would only go to make it a bit more of a problem
The fact that your friends can direct connect to you should give them that much more of an advantage over others, and if they know what time you will be switching on that too would be an advantage, but I like to think that the network is open to all users and whilst some more efficient way to discriminate against REAL free loaders (and not those we think are freeloaders) would be a good thing IMHO. I would not like to see the network used as a private means of exchanging files, which it seems you might be suggesting. I haven't explored the subject, but feel sure there are other ways to distribute files among a small group of people.
My answer is based purely on what I consider the Gnutella network to be and I would be interested to see what others feel.