Did you have any success? I had/have the same problem with an Wifi card, but using:
lxc.net.1.name: wlan0
lxc.net.1.type: phys
lxc.net.1.link: wlan0
helped to bring the interface into OpenWrt LXC, but there is still a performance loss and it doesn't feel like the "real solution". Speeds are...