Setting traffic class on return packets

I have a network topology: Server router1 router2 router3 edgeRouter "internet" All routers are linux based, and support iptables. The server sets traffic classes with iptables (--set class X:Y), and routers do some "routing" based on the class that is set. (Class depends on the originating application). The edge routers forwards the packets via our ISP to the internet, and recieves the return (reply) packets. The recieves replies ofcourse have no traffic class set. Is it possible to use an iptables rule on the edge router (mangle, or something simmilar), to track the return packets (NAT-style, packets from "ESTABLISHED" connections) and to mark the returning packets with the same traffic class as the originating packet? Enabling NAT on the edge router is not a problem. TLDR: How to use iptables to classify ingress packets with the same class as egress for the same connection.