Almost 7 years later and the documentation still states that customers will see a message saying that there is a maintenance (VCD 5.5) - but they still see "Service Unavailable".
I think you looked at the wrong date. vCloud Director hasn't even been on the market for 7 years. The original poster has had a forum account since then. vCloud Director's debut was in September of 2010, shortly after the VMworld Keynote speech.
As long as you are using a Load Balancer with the correct health check, this maintenance message will set the HTTP error state on the health check to something which should disable the node out of the load balancer pool. (HTTP Error State 503).
As @Gortee said, the load balancer pool can used to note that the site is down once all nodes have a bad health status ... instead of passing traffic. At least some can do this (i think), or would require a config change. If you are already doing a maintenance window, this should not add too much to the process.
In the case of a single cell environment, one would hope that the cell is not down for very long ... let alone to do work on the vCD application you have to stop the vmware-vcd-cell service which sets this message (service vmware-vcd stop).