You don't even need to do that. The ARP packets seem to be routed to the network through the normal route table, so replies for the wifi IP get sent via the eth

You don't even need to do that. The ARP packets seem to be routed to the network through the normal route table, so replies for the wifi IP get sent via the ethernet link and the network switches seem to learn that as the return path. As for rp_filter, it's set to "2" on my machines, which seems to allow return packets on the "wrong" port as long as it's an address that any other port owns.