I know that in command line it can be done:

neutron floatingip-create [-h]
                             [-c COLUMN] [--max-width <integer>]
                             [--noindent] [--prefix PREFIX]
                             [--request-format {json}]
                             [--tenant-id TENANT_ID] [--port-id PORT_ID]
                             [--fixed-ip-address FIXED_IP_ADDRESS]
                             [--floating-ip-address FLOATING_IP_ADDRESS] //например
                             [--subnet SUBNET_ID]
                             FLOATING_NETWORK

I tried to pass a single ip address to os-floating-ips-bulk POST
http://developer.openstack.org/api-ref-compute-v2.1.html#os-floating-ips-bulk-v2.1 (http://developer.openstack.org/api-re...) body:

{
    "floating_ips_bulk_create": {
        "ip_range": "10.10.59.176",
        "pool": "ext-net",
        "interface": "eth0"
    }
}

but it did not work