OS::Neutron::FloatingIP has a floating_ip_address property that you can use to get a chosen floating IP address. However, this is restricted to administrators in the default Neutron policy. In general, cloud tenants don't get to dictate to clouds what resources they are assigned; that just isn't a robust way of building a multi-tenant system. I'd suggest that you're doing it backwards, and you should be telling the server what floating IP address it got assigned by Neutron rather than the other way around.
In case that doesn't make the question moot (i.e. you're an admin on a private cloud, you don't care about interoperability, and you're determined to do it this way around) then the easiest way is probably to create a software deployment that cats the file to its output, which can then be accessed through the deploy_stdout attribute.