Wouldn’t it just be easier to have a pre-routing rule that looks at the destination IP and port (80) and sets the fwmark associated with routing to your gateway of choice? If rapidshare uses more than on IP address and they are all in in a small range then you could open up that IP address to be a subnet.
This list http://forums.peerblock.com/read.php?12,3963 seems to be quite valid.
It is possible to enter all ip ranges in net balancer config but even so, only rapidshare will work. Some forums will also need the ip address to be “static”. I think it would be smarter to have a way of choosing the gateway per request.
But is this even a iptables/routing question (as in “Can it do it”?) or do I need to consider somekind of proxying?