Isn't messing with TTLs a pretty obvious thing to do when communicating covertly? And wouldn't that kind of packet inspection only catch cases of TVs literally acting as packet-level routers, but not catch application-layer shenanigans like aggregating statistics in the exit node so that all the data going over your own network appears to be originating from the same device?

It's still implausible, but thoroughly verifying that your TV isn't spying on you isn't trivial.