Tcpdump/Iptables on bridge interface without assigned IP address
If you can ping, there is traffic in your bridged interface. You can watch the traffic using the appropriate options of tcpdump:
tcpdump -elnXXi br0
In the same way, you can totally filter this traffic using iptables, taking into account that it will only operate to the IP layer. You would need ebtables if you need to filter traffic on layer 2 (Ethernet)