In this scenario, I have a machine that boots its root volume off of iSCSI, and it only has a single network interface. If network configuration is done the normal way, then taking the interface down to add it to the bridge would kill the connection, and the machine would hang since it would lose access to its root device.
Setting up the bridge in the initramfs pre-boot environment would seem like the right approach here....
Recent versions of CentOS/Redhat support this in dracut with the "bridge" kernel parameter, but I am unable to find the equivalent way to do this for Ubuntu Server.
Ubuntu's default busybox-initramfs package does not include brctl, so it looks like everything would need to be set up manually....