Dear Gerard,
Our apologies for the late response. Perhaps your question has already been answered through other means. But in case it is not, the below information may help you further.
You are correct that using a fixed IP address will not have HCP load balance across the two controllers in the HCP S node. You can only specify one IP address and hence, target only that controller.
HCP is relying on the DNS to round robin the two IP addresses specified in the DNS for the full qualified domain name of the HCP S node. The instructions for configuring the DNS can be found in the HCP S node documentation.
I also want to mention to specify the HCP S node two virtual IP addresses (VIP) in the DNS configuration, not their physical IP addresses. The use of the virtual IP addresses ensures high availability of the HCP S node in case one of the controllers become temporary unavailable.
Thanks,
Ab