I wrote a previous blog post about the HP z27q detailing the hacks I had to use to get it work with my Macbook Pro.
Well, at some point with El Capitan 10.11.1 or 10.11.2, Apple added a profile for this monitor. So it now works without any modifications to the OS.
The experience still isn’t completely seamless … you probably won’t get a boot screen logo when you start the machine, and if you unplug your laptop frequently, it’s easy for OS X to get confused. But, in general, the HP Z27q is now the most cost effective 5K monitor on the market for OS X.