Despite the limits of their 400 nit screens, Atomos uses a firmware update to add HDR functionality to their Shogun and Ninja-Assassin.
While the full technical details of HDR are still being ironed out between standards groups and manufacturers (hybrid log-gamma vs. DolbyVision's Perceptual Quantizing, for instance), one thing that is pretty universal to the platform is a screen capable of a high level of brightness, generally 1000 nits or more. Atomos has come out with their Flame and Inferno lines of products, which have screens capable of 1500 nits, which offers the ability to preview the full brightness and contrast range of HDR imagery on the monitor.
However, Atomos still has a large number of their Shogun and Ninja Assassin recorder/monitors out in the field (while new Ninja Assassins appear to be off the shelves, you can still by new Shoguns at B&H). While some companies are notorious for leaving early adopters out in the cold (Apple being the most egregious on this front), Atomos has brought limited HDR functionality to the Shogun and Ninja Assassin through a free firmware update. This update also brings beta support for CinemaDNG raw capture.
To be clear, the screens themselves retain their 400 nit brightness limit: Atomos hasn't magically gone back and made the screens brighter, and thus better able to handle the wider brightness range of HDR. But they have built tools into the firmware to enable directors, DPs and producers in the field to better monitor their HDR footage and make more accurate exposure decisions at the moment of capture. Unfortunately, this doesn't magically make a camera that doesn't have a Log/Raw capture setting suddenly capable of HDR imagery, but if your chosen camera can capture wider latitude images (unlike the DSLR in the image above), this upgrade is a welcome improvement.