I'm looking for a way to configure the following command line via systemd's networkd:

  ip addrlabel add prefix 2001:1111:0:222:33::/128 label 666
  ip addrlabel add prefix 2001:1111::/32           label 666

My suggestion would be the following addtion to /etc/systemd/network/foo.network
 
  ...
  [address]
  address = 2001:1111:0:111:44::/128

  [address]
  address = 2001:1111:0:222:33::/128

  [addrlabel]
  prefix = 2001:1111:0:222:33::/128
  label = 666

  [addrlabel]
  prefix = 2001:1111::/32
  prefix = 666

Background:
When searching for a source IP, the Kernel prefers those that have the same  label as the destination network. Packets to 2001:1111::/32, which is labeled with 666, would thus originate from 2001:1111:0:222:33::/128, which also has the label 666. All other IPv6 targets would have the default address label "0" and thus be originate with the address 2001:1111:0:111:44::/128 which also has the default label "0". This is useful to have a static IP for company internal targets and a temporary random address (-> IPv6 privacy extension) for everything else.

Current workaround:
Using a service unit that waits for the interface to come up and then executes /sbin/ip as suggested in http://lists.freedesktop.org/archives/systemd-devel/2015-April/030725.html