Remember that on a network that's connected with Ethernet switches, you won't see most of the data even if the network card is in promiscuous mode unless you have a managed switch with port mirroring.
One way to achieve what you want is to use an arp poisoning tool, such as Ettercap. You should be able to then apply netfilter rules or tcpdump the information you require.
Edit - More detail on request of OP:
From your question it sounds like you wish to snoop on passing network traffic and pass it through iptables.
By using a bridge, you're able to achieve this by sitting between the interesting machines and their gateway, physically, but it sounds like you would like to do in more passive fashion, hence why you're trying to use promiscuous mode on the NICs. Right? (If you don't know, promiscuous mode allows you too see Ethernet frames that were not addressed to your NIC)
The problem you have is that an Ethernet switch is designed so that it learns the MAC addresses on each port and uses this to "route" Ethernet frames to the correct port based on their MAC address. This is to reduce collisions associated with Ethernet hubs (something you rarely see these days). Therefore, you will only see Ethernet frames destined to or originating from your NIC including broadcast Ethernet frames, such as ARP, but not foreign traffic. This is a good thing :)
Most managed switches (not a dumb desktop one) allow you to designate a port mirror so that all Ethernet frames are replicated on a specific port where you can attach a machine in promiscuous mode and capture "foreign" Ethernet frames using tcpdump/Wireshark.
This still won't let them be captured by iptables, however. So you need a way to act as an Ethernet bridge again between the interesting hosts and their gateway but without being physically in the path. Enter Ettercap which is an arp poisoning tool. It tricks your interested hosts (and the switch) that your machine MAC address now owns the IP of the old IP gateway by sending out a "gratuitous arp". The interesting machines will unwittingly send all gateway/default route destined traffic to your machine. Your machine will now forward packets through its IP stack as if it was the gateway.
Where this won't work is when "port security" has been enabled on the switch, a not uncommon practice. This is to stop arp poisoning by blocking gratuitous arps where an IP is moving from one Ethernet port to another.