Is that new in 1.6? As of 1.5.11 there was a lot of hacking required to get a true zero downtime reload: http://engineeringblog.yelp.com/2015/04/true-zero-downtime-h...
The DNS resolution work that is going into 1.6 releases may lead to a workable approach where you over allocate servers in a backend and then update them to have the right host/port. Last I checked, however, you still couldn't set the port dynamically.