We require a system similar to LVS-TUN that will allow to have several servers hidden behind a load balancer. This cannot be achieved with NAT, because the servers will be located in different datacenters. And, the return data (server -> user) must be direct, not going through the load balancer.
We have tried to implement LVS-TUN, but it seems somewhere in the network there is something blocking the IPIP traffic.
So we think the ideal solution is to have the hidden servers spoof the source IP addresses to be the same as the load balancing server, and that server will redirect all responses back to the correct server, as it it had been a direct connection from the user's machine.