I doubt iptables alone would work, if the kernel isn't compiled with the CONFIG_NETFILTER options on. I should give it a try...
uberoid comes with iptables compiled to android froyo, and a gcc and other utils too, it should work. or use a real iptables from a linux chroot running on android. i'm going to make up an image for debian for the rena that includes the armv7 gcc.
other tablets come with it too, just take the exe it should work.
otherwise just compile a static linked iptables from within the chroot or a real linux system or maybe even the ndk to arm, that will work too.
however, iptables is just a firewall, it does not make vpns.
for that you need openVPN.
If iptables worked, I'd install my company's Cisco AnyConnect VPN, which would do the job for me.