freebsd bridging and failover

[Val P]

Is there a way to implement a failover in a bridge? Basically I'd like to keep two 
filtering bridges on the network at the same time, one primary and one backup. 
However, since freebsd bridging doesn't support STP, I can't do that without creating
a loop. Is there something (like the vrrp implementation but for bridges) that I can use?
 
Theoretically I can interconnect the two bridges over a serial port and write some sort
of heartbeat application, but I'd rather have a solution that has already been tested.
Also, my simplistic solution would cause existing connections to be disconnected
during the failover.