Fabio has just advised:
Hello, the first plugin for Nikon is available at the Astroart web site:
The cameras supported are those with Bulb control via USB, (since 2013, so theoretically: D5200, D5300, D5500, D5600, D7100, D7200, D600, D610, D750, DF, D4S, D500, D810, D810A, D5, 1V3). Focusing requires the Camera Interface version 6.01. It's compatible with Astroart 5.0 too (without focusing).
The main problem during the development was that every camera requires a different DLL (this is installed by the Setup program) and possibly a slightly different control.
Some cameras may require to install the Microsoft Visual Studio Redistributable 2013, available at: www.microsoft.com/download/details.aspx?id=40784