How do I untag traffic leaving a certain ethernet port without using a VLAN-aware switch?
Network diagram:
Let’s suppose that you have a 2 port routerboard. The eth1 has 2 VLAN: vlan1 and vlan2.
You create bridge1 and bridge2 and assigne to the bridge1 the port vlan1 and to bridge2 the port vlan2 (NOT the phisical port eth1).
Now if you want to untag the vlan 1, you have to simply add to the bridge1 the phisical port eth2.
Thank you very much for this informaton. 
Maybe it’s time to include this in the user manual. 