Haven't read the word "Zwieback" in years. Makes me think of my Grandma. She made me Zwieback with butter and sugar when I was little. Quite disgusting - but nostalgia always wins!
Well, that's the point, isn't it? Hardtack gives your crew a -1 morale and with pure zwieback it's pretty much the same. ;-) Didn't hear of the butter+sugar version though. Zwieback was given to us kids back then, when we had serious digestive issues... Perfect for journeys on open seas, too, was my thought.
But I think for this recipe you can use any kind of tasteless cracker.