Finally, after all the waiting and poking around there is now an official guide for using a Nexus 9300 as HW-VTEP for NSX.
The recommended NX-OS release is: 7.0(3)I6(1) or later in same main release
The supported VMware NSX release is: 6.3.3 and later on the same main release
I hope i have some time in the coming days to deploy it in my lab and test it. But after a first overview the guide look pretty forward.