Iptables is be able to run in a stateless NAT mode if you disable the
contrack module:
http://jonatan.nilsson.is/running-a-semi-stateless-linux-router-for-privateā¦
It might be worth noting that iproute2 can provide a stateless NAT mode
w/o IPTABLES which might be perfect for forwarding the IPIP traffic
http://linux-ip.net/html/nat-stateless.html
--David
KI6ZHD