I learned something and it's a good thing to put in my memory bank. I would therefore use Sam's method of weak cool nitric. I would give it a good stir every so often. The nitric will passivate the Kovar once the Ni is gone. The 80/20, Au/Sn eutectic melts at 280C (561F). They applied the braze a little hotter than that, but not much. That's not very hot but, in the middle of the lid, the Ni would be the thinnest and I can see the the braze penetrating the Ni there. No type of electroplating is uniform on a part. For the lid, the Au and Ni will be thickest on the corners/edges and thinnest on the center. The definition of eutectic is that particular alloy (in %'s) that has the lowest melting point. For Au/Sn, it is 80% Au/20% Sn.
What strength HNO3, Sam? How long does it take?