How do I launch a VM at foxtrot with both public and private ip

I realize that foxtrot has a limited public ips for users but my experiment requires more vms than that. so I am planning to launch many worker nodes with private ips and a head node with a public and private ip as well so that I can use the head node to connect the private network. However, I see that vws.metadata.association only allows you to use 'public' or 'private'. Does anyone have any solution?