Using VCO to assign external IP addresses on vApp deployment.

A while back Christophe wrote an article on the vcoteam site explaining how to use the AMQP integration to extend vCD functionality in VCO (link here: Extend vCloud Director with vCenter Orchestrator).  One of the example use cases listed there was: "Getting IP Addresses from an external System and use them for provisioning the VMs of a vApp".  Has anyone actually implemented this use case yet?  I know the "external IPAM System" integration piece will be different for everyone, but I'm interested to know the best way to intercept the external IP address allocation of vCD on a vApp deployment and "replace" it with the desired IP address.  Is there a way to edit the params used by the deployment task on the fly, or do you have to let vCD assign one the normal way first and have VCO come in afterwards to change it to the new IP?