What would be hard is also making sure that packet sniffers in the middle wouldn't be able to detect it. Specially with all the varieties of router hardware. Are we going to have a backdoor in all of them that prevents passing on that data?
There's still the question of packet sniffing by an intermediate device. The attacker would need to control (the network interfaces of) every device in the chain, use the ability very rarely to avoid detection, or hide data in packet metadata that is later decoded by interception equipment. This third option is probably fairly straightforward on any NIC with TCP checksum offloading.