... but, mostly, why do you care if they are using your textures?
... and why are you so concerned about learning how to force a DL on clients?
I don't think you have a clear enough understanding of what free and open-source means, or of the related issues (like, Vista is not the only OS out there), to be trusted with such information, even if such a method does exist.
I, for one, would not be interested in playing such a server.
I am the client owner and if I connect to a server, I prefer to decide what files to put where on MY machine.
/disconnect