I use vyos instead of OpenWRT, but I'd presume OpenWRT can mirror a port? It'd be better to do it on your switch of course. But you could mirror your traffic going across the LAN-WAN barrier and direct it to a security onion install, it's an opensource IDS. It has pretty heavy demands, but traffic analysis is not an easy, computationally cheap task.
I guess the big problem here is analysis, because a modern home network moves a massive amount of traffic, to many endpoints.