Have you tried modifying the TTL on the router? I can't give you all the information but it's a place to look. I believe dropping it by one is what's needed. I've used a Belkin router flashed with padavan firmware and it has an option to adjust TTL, just a checkbox, but it seems to enable unthrottled unlimited tethering as far as I can tell.
I believe you should add one, not subtract. E. g. if TTL on the phone is set to 64, you'll want router to send packets with TTL 65, so that one extra hop (from router to phone) brings it down to 64 and the carrier can't tell the difference.
