It's highly unlikely that you will be able to do this.
The architect is designed so that when your using NAT the packets are rewritten by the load balancer and forwarded onto the server using the load balance method algorithm your first selected when your compiled the kernel.
The advantages are you don't need as many IANA assigned addresses.
The disadvantages are that the load balancer can only take about 9mbit of data a second.
The IP tunnel option works with a different method, i.e the returning packets don't have to go through the load balancer but directly back to the requesting system.
The advantages are that it's more secure within the internal segment of the network. "no change over the internet as packets are unencrypted"
The disadvantages are that you need more addresses from the IANA.
Also the processing power on the end servers is higher due to the encryption/decryption.
I suggest that you contact
http://www.linuxvirtualserver.org/mailing.html
And ask the guys who specialise in the software directly.
/Raz
