Hello,
I see there were some discussions and proposed implementations on
https://blueprints.launchpad.net/neutron/+spec/auto-associate-floating-ip
blueprint (including a mail thread
http://lists.openstack.org/pipermail/openstack-dev/2013-November/019489.html
) but there was no activity on it
You can try and reach out the original assignee (sweston) on IRC.
I haven't heard from him in a while; if he's not actively working on this
topic anymore I think it's ok for you to take over.
Feel free to get in touch with me (salv-orlando) on IRC for discussing
implementation alternatives.