Throttling NAT interface

[Casey Scott <casey@phantombsd.org>]

The gist of what I need to do is restrict the rate of
 off-network traffic coming in through a host. The host
is providing basic NAT to an internal network. I have
gotten pretty close to what I need to do with iptables
 and tc. The problem is that when an interface is throttle
 with tc, the source of the traffic doesn't matter. I don't
 want to throttle local traffic, just traffic coming through
 the machine from a WAN. The next step was to add another
NIC to the machine. Something like this:

eth0:  <local IP1>
eth1: <WAN IP>
eth2: <local IP2>

The problem is that even if  traffic destined to be routed
off network comes into eth2, which is throttle via tc, the
return traffic comes back through eth0. Since tc (tbf filter)
just controls the transmitting of an interface, I need to
force the NAT traffic to use eth2. Traffic that is meant to
stay local can use eth0. Is possible to do something like this?
How can I this host to only eth2 for NAT even though both eth0
and eth2 are in the same network?  eth0 is not throttled, which
 is why local traffic needs to use it.

TIA,
Casey