With a lot of luck and the generosity of a friend, I got my hands on a Nikkor AF-S f4/600mm. I tried it this evening, attached to a Nikon D810, on some Black-winged Stilts in the Neusiedler See -
Seewinkel national park. Not too easy to handle (it's so heavy!) and especially tough in low light, since it lacks a vibration reduction, but still - I'm very happy!