Comment 6 for bug 1736975

I've figured out how to work around this. It's all documented at http://djanotes.blogspot.com/2018/04/anonymous-bridges-in-netplan.html.

In short, I tell systemd-networkd directly to bring up the bridge. To do this, in addition to the rest of the configuration in netplan, I create /etc/systemd/network/br0.network with the following:

[Match]
Name=br0

[Network]
LinkLocalAddressing=no
IPv6AcceptRA=no

This leads to the interface being brought up and not getting an IPv4 or IPv6 address.