hi Christian,

not fully openstack compiant implementation, but. you might make it working by:

 you can create a port with several IPv6 addresses (so it would be reserved in OSP) and port security disabled for that port or network/subnet (this will allow to reach your vm on that port without filtering "spoofed" traffic which will be in case of packets addressed using the ip addresses which are expected to come only FROM  your port but not into it).

Create port and later assign some ip addresses to it then again some, then again... 5 in a go to ne safe, then sleep 60 sec and again openstack port set fixed-ip or smth like that... set it in a cycle :)

Then that vm, will have exact ip addresses "by accident" they might be in a smaller subnet :)