I know the IP address is built-in, but if you could set your IP-hiding client to only reply to incoming queries from other users of the same IP-hiding client, problem solved.

IP addresses should not be returned with query results, end of story. I don't understand why all gnutella clients are built this way. Using WinMX and connecting only to the WPNP network (not the OpenNAP stuff), the program knows the IP address of the person you're downloading from, but there's no way for you or the RIAA to see it. That's great design and should have been common sense for the architects of the gnutella network.