In practice, the best way to manage this is to have two hostlists. One that you receive queries from (as today) and one that you send searchs to.
This new list should always become better as you drop those that do not respond to your searches and/or som automatic test-searches performed by the client, based on your shared directory.

As for the freeloaders, noone would want to use them on the list over hosts to send searches to. But that is good for them, less traffic over their modems.